1-Here come the cops!
Because everybody, including most users of magic, quickly got fed up to the back teeth with malign mages and wicked witches, the magic users themselves have formed a police force to take down the outlaws and if necessary take them out. It can be proud of itself and out in the open, such as The Upright Society of Civic Wizards , in which case people who unfairly end up on the wrong end of a nasty spell or curse have someone to complain to, or it can be more of a secret society that quietly takes note of troublemakers and deal with them just as quietly behind closed doors. Either way, it makes sense from the point of view of the magic users to police themselves rather then have others force policing on them.
For every magical offense there is a magical defense in the form of charms, enchanted amulets and protective spells. Some, the cheaper ones, are relatively weak, others are the magical equivalent of an invisible suit of full plate armor, and even the strongest spells just bounce harmlessly off them or even ricochet on the caster. Perhaps graves are made necromancer-proof in this way. By itself, this would not deter magic users that are really out to cause trouble, however, if coupled with one or more of the other things on this list, it is a major reason why most magic users don't attack people randomly with magic. If they know that they can't just fry someone for sure with a sneak attack, that in itself puts off some would be troublemakers.
3-I don't know your Name
To cast a curse or indeed any spell directly on somebody, you need to know their true name and work it into the spell or curse to make it work. Otherwise the magic won't work. Therefore, at the age of thirteen everyone gets a second, secret name, given to them by a trusted wizard or witch. It could be that a lover or a close family member or a best friend is later given the secret as an ultimate sign of trust, but other then in those cases, everyone else is given a use-name for everyday use. Even a Judge cannot legally force somebody to give up his or her true name. Only a great deal of hard work can winkle out someone's true name and most who want to either don't have the skills or don't have the time or just don't want to use torture and can't think of another way to get it. Without the true name, the only nasty magic that can be done is chucking something like a fireball in someone's general direction. With it, the most unpleasant curses can be cast on someone by any magic-user with enough skill and malice.
4-Be careful, young Padawan, do not turn to the Dark Side.
To properly learn magic with any degree of skill, wizards and witches need to be carefully trained by other wizards and witches who have honed their power through experience and a great deal of practice. The most powerful users of magic could be compared to black belt karate masters. It is a rare karate master who will teach karate to those who he or she knows are going to misuse it, and it is even more so with magic that can be so much more dangerous. Of course, there are a few masters of magic who feel so unappreciated by society that they turn bitter and twisted and start teaching people to use evil magic. These can be a real problem out of all proportion to their numbers and, if not quickly dealt with, the problem multiplies as their students become skilled enough to teach others in their turn. Perhaps the PCs are sent to deal with one of these rogue masters of magic and nip the problem in the bud before it really gets out of hand.
5-You have to prove your worth
Black magic, real black magic such as zombie-raising and curses that can starve a well-fed man to death or rot flesh off the bones of living people, can only be done by making a pact with the Dark Elder Gods, and they sure as heck won't accept just anyone. After all, they have a lot of people to choose from and they only want the best of the best. To make contact with them, you must prey to idols of them on a certain night of the year,and the God will look you over in your dreams and read your thoughts. Most would be users of dark magic are rejected at this stage, others fall flat when they back out of dedicating their souls to the Elder Gods in the dream. Those that pass the test are given basic skills in Dark Magic, but must practice them over many years to gain their full powers. Once they dedicate their souls, they cannot back out. Any that do, find their own dark spells cast on them by the thwarted and angry Gods that they cheated, and when they die the Gods eat their souls.
6-Wanna be in my gang?
Dark magic and the knowledge of how to cast it is limited to four large, powerful and immensely dangerous gangs with fearsome and well earnt reputations, and they guard their secrets jealously. If they find someone else using dark magic, they invariably do one of two things. First, if they like the look of the person and think he (they don't accept women) has what it takes to hang out with them, they will make an offer to join their ranks. For those who do, the gang becomes their life. They start as a prospect, and do all sorts of things for the gang,some legal, some not,and are taught certain spells and given a symbol that other gang members recognize. Depending on your world, the gang can be like the Mafia with few or no identifying marks, or it can be like an outlaw biker gang with patches on their back, riding horses, broomsticks, flying carpets or, in a world with technology as well as magic, motorcycles.
After a few months as a prospect, the gang votes on full membership for the new recruit. As either a prospect or a full member, the recruit must obey the gang laws; no talking to the police or equivalent of the police, if such a thing exists; no leaking the secrets of the gang to any non members; no using hard drugs; no cowardliness; kill if need be for the gang, always respect fellow gang members. Informers, cowards and those who try to leave the gang will be murdered if caught. Once inside, the gang serves as your new family and monopolizes your life. On the plus side, it protects you from others and helps you get rich, which is it's main purpose.
If the gang does not want to recruit someone, or the would be dark mage turns their offer down, the gang will demand all the mage's or witch's dark magical books, scrolls, wands and the like. Failure to give them up at once will result in a severe beating with both dark magic and physical force before the items are taken. Going to law enforcement to complain, or gathering new magical items and continuing to cast dark spells, means the gang will murder the caster if it ever finds him or her again.
7-Geas of Goodness
All mages and wizards have a geas cast on them when they first start learning magic, that stops the vast majority of them from using their skills for evil. Only the very strongest of them have the sheer magical power to break the geas and not many of them who reach that level of power want to. The flip side of this is that whilst there are fewer bad magic users, the ones that are bad are very powerful and very very dangerous. PCs beware.
8-Without the bling, you can't cast a thing
To cast magic of any level of power, you need a small piece of tassite or other magical mineral. Without this it does not matter what level of magic you have learned, you cannot use your knowledge. Because of this impediment, wizards are rare and it is a lot easier to keep magical power out of the hands of the evil, unpleasant, or insane. But the higher levels of the criminal underworld have some of this mineral in their hands. Evil magic users are likely to be linked with the world's equivalent of the Mafia, as the head of a crew of non-magical but muscular hoodlums.
9-Only in the Henge
Magic only works in a handful of areas in a country, be it old stone circles, royal barrows, or the world's equivalent of Yellowstone or Ayers Rock. If there are police or the equivalent, these places are guarded with care; even if there are no police, the army will guard them and/or a royal palace or government building will be built around them. The government does not want just anybody casting magic willy-nilly in such sites, especially as the effects of the magic are greatly enhanced. A necromancer casting here could raise an entire Undead army or the bones of a Dragon to carry out his or her will. If a country plunges into civil war and becomes your world's equivalent of Somalia, there may be no police, army or government left to guard such a sacred place. Even then, one of the warring militias will most likely try to take it over for their own use. Which is when it is most likely to fall into the wrong hands. Wars between nations have been fought for the possession of these sites of high magic.
When a person casts a spell, their hands glow for days afterwards, brightly enough to serve as torchlight in the dark and only be blocked by the thickest of gloves. Good or neutral spells shine with a soft green light, but evil spells shine with a baleful red light, and depending on the power of the spell it takes anything from an hour to two whole weeks for the caster's hands to stop shining. Good or neutral spells are rarely a problem for anyone but the most magic-fearing government, but evil spells will mean that law enforcement and/or vigilantes will soon have a good idea of who cast the spell, especially in hot weather when thick gloves will be just as incriminating as the red glow.
Mages and witches are so powerful here that they extract protection money from those without any access to magic. This in itself however restrains them to an extent. If a magic-user goes rogue and starts using evil magic despite being paid off, it will cause the non-magic users to panic and flee the area. Other magic users will end up intervening in such a case and tell him or her to knock it off, or else. They don't want to cause their prey to flee the area or get so desperate that they end up rebelling after all. Magical violence must have a reason behind it.
Even *good* magic such as healing or neutral magic has a chance of going wrong, but with malicious magic, the chance of it going wrong shoots up exponentially. Fireballs or lightning bolts risk severe burns of the mage's hands. Curses often bounce back on the caster and affect him or her just as badly as the target. And yes, it is possible to raise the Undead or summon a demon, but few are the magic users that can truly control their new Undead army or get what they want from the demon. So few in fact that they are almost legendary. So unless enraged beyond reason or in a really tight spot, mages and witches do not cast malicious spells on people, certainly not on a mere whim. If you have angered a magic user to the point when he or she is casting foul spells at you, then that magic user wants you dead and really does not care what needs to be done to achieve that goal.
13-Magic itself is illegal
Magic of all kinds, not just the evil variety, is illegal. This may be because of the state religion being against it, the nobles being against it in case it tips the balance of power, or just the people in general having a partly unjustified fear of it. And the state is both able and very willing to seriously enforce the anti magic laws as well. Even good or neutral spells are hard to get, and for the nasty stuff, you need to risk dealing with certain sections of the criminal underworld, who are both suspicious of potential undercover police, and not nice to deal with. Bad spells cannot be fully wiped out, any more then it is possible to get rid of every single gun by making gun ownership illegal, but it is impossible for most people to get their hands on them.
14-Everybody knows magic
In this country the rulers have taken the opposite view to the problem of magic getting into the wrong hands. Everybody is taught magic from when they are very young, and mages and witches just have learned a lot more magic then anybody else. Even a mere beggar most likely knows a spell or two. And because everyone knows magic, anyone wanting to use it to cause harm on someone knows that their target, and possibly any bystanders as well, are likely to react by casting spells at them, resulting in Mutually Assured Destruction if things get out of hand. Of course there are a few outlaws who don't care about that, but most of them end up dead if they act up in a town or village.
It does not matter how much the magic user has trained in their craft, magic used to hurt people or animals always bounces back on the caster. So killing somebody with magic will kill you as well if your spell is successful, whilst using magic to ruin somebody's life will ruin your own. Since magic users are rarely willing to go that far, evil spells are rarely used. There are loopholes however; certain cults can use one of their own members as a scapegoat for their leader. They don't do this often though as it would quickly reduce the cult's numbers.
16-The Gods Will Intervene
It is widely believed by most people that the gods themselves will intervene if magic is used for evil. This could be the literal truth, only true in a very few cases, or just propaganda put about by the ruling class to stop people from casting bad spells. The fear of godly retribution keeps a lot of magic-users on the straight and narrow path.
17-Only Demons can cast Evil Spells
Humans are literally unable to cast evil spells. Even fireballs fizzle out if used against a human or animal or for arson, and anything darker used against someone is impossible. There is a loophole, however, in that it is possible to summon demons from the Abyss, and they can cast black magic for you without any problems. The problem is persuading them first not to attack you as soon as you summon them, and second, to attack your target for you. Rare is the demon whose price is not too heavy for most people to consider paying. When someone skilled in the magical arts has summoned a demon and persuaded said demon to do their bidding, normally it's because the target did something truly unforgivable, like murdering the magic user's entire family, and this is the only way to get revenge. Of course, the law takes a very dim view on those who summon demons.
If you cast evil magic, no matter who you are and how powerful you are, you are going to Hell when you die. No amount of confessing your sins, donating to charity or other good things that you do will get you out of it. The only ways to avoid Hell's horrors are to avoid death altogether by becoming a vampire, a Lich, or stealing people's bodies when your own is getting decrepit, leaving them stuck in your own. And none of these ways of avoiding death is easy to do, . Vampires are hard to find and persuade to turn you instead of eating you or alternately may simply not exist. Attaining Lichdom is so hard as to be almost impossible. Stealing other people's bodies is slightly easier, but only very skilled magic users can do it, and even fewer of these are able to cast a geas on the person to stop them telling anyone who they really are. And even in another body, if that body should die for whatever reason, then the caster is still going to Hell.
19-Almost nobody knows magic
The rulers of this country have not only banned magic, they have banned education for all but the elite. Only the elite are permitted to possess books of any kind, magical or not. By keeping most of the population as illiterate labourers, the government stops the spread of dissent. A side effect of this is that is that there is are very few magic users of any kind, and therefore even fewer bad magic users.
Every time dark magic is cast at any level, it attracts the attention of the Things. These are not Devils, demons, or elementals, but completely alien creatures from other planes that intersect with our own. Dark magic twists the planes together for a short time, and these things of unknown intelligence can come out of it. The few that have survived the attacks of these beings describe great living gasbags propelled forward by frilly fins, with huge saw toothed tentacles that latch onto their targets and carry them to their large tooth filled mouths, where they are chewed up and swallowed. After eating the user of dark magic, they head back wherever they came from and the rift between the planes heals up. Low level dark spells won't always cause that to happen, but the effect is cumulative-every dark spell you cast raises the chance that one of those horrors will come looking for you.
And if you were to do something really dark like raise an Undead army, well, most of your undead will be torn apart before they can inflict enough wounds on the Things to send them back where they came from.
21-It rots your fingers
Casting dark magic with the fingers quickly causes them to rot and decay. It is possible to an extent to get around this by using wands or staffs but after only being used to cast one or two dark spells, it corrodes them and they become useless for casting anything ever again. And magical tools worth their salt are not cheap. The pain and/or great expense is enough to put most would be casters of dark magic off.
22- No Morals
In a certain country the only thing that is illegal (and punished with death if the secret police find out) is treason, be it against the ruler, the nation, or how things are ruled. Therefore there are a lot of bad, by most people's standards in other countries, magic users, but in that country their magic is not formally "evil" unless it is used to commit treason.
23-It's too expensive
Most bad spells originally had military uses and then the knowledge of how to cast them illegally found it's way to those who wanted to use them for crime or revenge. But if a country has been at peace for centuries, then the large amount of money spent on researching and making new spells is going to be spent on more useful things like schools and hospitals. So new black spells are not being made, thus there are fewer evil magic users, or at least they have a lesser range of spells to cast.
24-It drives you crazy
Unless the magic user has a very strong mind, every time a dark spell is cast there is a danger of affecting the mind in a negative way. Some have become burnt out at best and catatonic at worst, others have become too scared to go outside, still others have become obsessed with casting evil spells to the point where either law enforcement or vigilantes in even the most corrupt or violent country have had enough and take them down. The most dangerous of this type are those who have become obsessed with power.
25-It's the economy, stupid
Even allowing for the large sums of money made by extortion/making forbidden items using bad magic, in most countries the white economy is still bigger then the black economy. So for every mage-extortionist or necromancer, there will be many magic users protecting things, making things, finding things or strengthening things.
In the not so far away past the most dreadful things were done with black magic, from the creation of whole Undead armies that slaughtered everything in their path, to magical death camps, to the summoning of powerful and horrible demons, bringing black magic into disrepute with just about everyone, even organized crime. Black magic is held on the same level as having sex with small children or torturing harmless cute little animals. Not only are the laws against it ferocious and rigidly applied, but just as importantly, most people at all levels of society agree with said laws. In time as the memories of the horror recede into the past, this dislike will weaken amongst more and more people, but not yet, as what happened is still seared into the nation's collective memory.
27-Shake, Rattle and Roll
Casting evil magic makes the ground shake, and a dark spell of enough power can literally tear apart a city. And almost everyone in said city, from the rulers to law enforcement to organized crime to people in general have a vested interest in not having their city torn apart. Whilst the Law goes after said bad magic-users, the underworld in most cases denies said users the knowledge they seek, not because of morals but just because they don't want their own houses razed to the ground by an earthquake.
Casting evil spells makes you sick, and the more powerful the spell is, the worse the sickness will be, up to and including death for the caster as a result. The effects do wear off in time, if no further black magic is cast, but they are cumulative. And each time the person takes longer to recover from the effects, which include nausea and vomiting. Since magic users just like most people prefer to be well, they rarely indulge in bad magic, unless against people they really hate.
Using bad magic is invariably quickly fatal for the caster, so only someone who is suicidal and decides that they have nothing to lose will cast it, in a showy way. They may be ultra-patriotic at a time when their country is losing a war, deeply depressed, want revenge in a big way, or have their loved ones taken hostage to force them into it; whatever reason it is, it won't be cast lightly. Magic users who use black magic are the equivalent of suicide bombers.
The use of black magic causes bad luck and angers The God of Small Things. Again, the effects build up over time, from little things like the button on one's trousers popping or getting caught in the rain, to a handful of cells in one's body turning cancerous-and if not quickly dealt with, metastasizing all over body and slowly killing the bad magic user. The latter only tends to occur after decades of casting dark spells.