Yet another exploration of a weird idea, let's see how you like it. Still incomplete.
Imagine a people whose moral code is largely based on taboos. Not that they are on a large-scale forbidden from certain things, but they rather choose their own taboos. The general opinion of many is, the more immoral things/deeds/sins one avoids to do, the more of a moral person is it.
Initial notes: - the society could be relatively "young", the barbaric/nomadic roots still play a large part, while the new settled lifestyle offers new options and challenges. Clearly, the need to preserve traditions and culture is very strong.
- as I picture it, the area this society lives in is more than just a village (if everyone knows everyone else, much of the potential is lost), but likely not larger than a city-state, or a few allied towns. If a large-scale application is desired, there has to be a strong supporting religion and philosophy. The nation is quite spiritual anyway (and shamanism may be still present), but an overstretch could break the whole system down.
Self-imposed Taboos A taboo in this context is something the person chooses to avoid for the rest of his/her life. While there can be in theory any number of weird principles, it is mostly referred to only a few key moral ideals. There is only a limited set of them (exact list still worked at, but likely to resemble The Ten Commandments in the end, duh), the base of the whole society.
Examples:
- killing
- stealing
- lying
- "devotion"
Some taboos are not negative, but also positive; in other words, they are not the ultimate restriction, but rather the ultimate command. Note that this can produce many problems, for Evil can easily come from the over-eager pursuit of some ideal. The Devotion taboo can easily fall prey to this; even if you "only do the best thing" for the beloved person, or in some cases country.
Spiritual and idealistic, the society can be but very pragmatic. Warriors may take life of others, so indulge in violence, but must be loyal to someone. They are allowed to kill, for the wellbeing of society. Spies may cheat or steal or what, but must also have some principles.
As for any general taboos for everyone, an adult local _must_ have some taboo, if he wants to live among others.
Tatoos Thinking of some way to mark these chosen taboos, it always came back to tatooing. I have somehow little love for tatoos, but I find no other simple means to mark this permanently.
But, tatooes are useful. Especially if you break your taboo, as they can be "erased ", never to be tatooed again. (May conflict with some powerful magics though...) And as they would be probably on visible parts of the body, people would know who they deal with.
The Double-sized option To prevent certain problems, and the absoluteness of some commands, the taboos (and tatoos) could be in two "grades" or sizes, or what. As for killing, the lesser taboo might only forbid murder, or any intentional kill (but might allow killing in self-defense). The greater taboo forbids killing, or maybe even harming any living beings (the buddhist option). Not to lie, can be in the common sense (a little lie to save your life _might_ be accepted) versus a complete honesty in all matters.
Laws There surely exist laws. While in some ways they may overlap (stealing, murder, etc is against the laws, and will be punished appropriately), the taboos most often exceed the behaviour required by the laws. If someone breaks the taboo of Devotion to an individual, there is no punishment (from the law at least).
Where laws and taboos overlap, strange things happen. If someone is a thief, he won't be punished that hard for stealing, if caught (but of course is watched by everyone much more!) To break both the law and a personal taboo calls for much more serious punishment (plus see below the breaking of a taboo).
Rulers must have one major loyalty: the Devotion for their country. While other taboos grace a ruler, many choose nothing besides this one.
Breaking and abandoning a taboo To
break a taboo means to violate the given promise, of what cause ever. If it was a crime, it is punished by the law accordingly. But the respective tatoo is branded, a painful process that leaves permanent scars. It is rare, but an infection has occasionally killed the unfortunate.
Abandoning a taboo means, that an individuum decides to act against it. Rather than break the principle, he or she asks for the taboo to be removed, and most often chooses another one. The beautiful fluid drawing gets additions that break the patterns. While tolerable to look upon, everyone recognizes it.
Both are considered immoral in a way, breaking is much more serious. As with most crimes in most societies, if people know the circumstances, they _might_ tolerate the unfortunate (killing to save own children may be an example). Others may be willing to hear the story, but also may not waste the time. But generally, these people are ostracised from the society in some way.
Giving up a tatoo is a bit more honorable, because you anounce to everyone that knows you, that you change your morals.
Strangers vs. locals Now, enter PCs from another society. Without any (correct) tatoos. The people probably know of foreigners, but the immediate shock of people without ANY morals would be great, and will provoke a lot of roleplaying. Be assured that some mothers will grab their children and run away nonetheless. Shopkeepers will be wary too.
Strangers are those that have no tatoo (decorations don't count, it must be the real thing). While somewhat tolerated, if they want to settle down permanently, they must also choose one. Locals without a tatoo are banished; the youths must decide for at least one tatoo until a certain age, or leave forever.
Interestingly, this relic from primitive times has with the population's growth produced a large bureaucracy, that keeps record of dates of birth, and the people's individual taboos. Among other things, this may cause in times of need (like war) a call for "proper individuals". Anyone without a taboo against killing could be conscripted into the militia. Famine approaching? All that can steal could be sent in a raiding party, etc. While rare, it has happened a few times, and is another reason why to have at least some moral principles. Outsiders that wish to stay for some time, may also be subject to this, espacially given the fact they have no true morals...
Note, however, that if someone looses their last taboo, he or she would have to choose another, or be not considered local anymore. A few criminals have already been banished, or killed if they resisted their "removal".
Note also, that to feign a tatoo is a very serious crime, and a deeply immoral deed. The society is in a way built around this basic requirement of honesty from its members (is not considered as a part of the taboo against lying). Should this cornerstone be somehow undermined, chaos is likely to result.
General mindset? Well, people are serious about it.

The thing is, if you _really_ decide for something, you must keep to it, or feel the results for the rest of your life. Even if you decide to never break the taboo again, the mark is gone, and your failure is marked. (There exist numerous legends of people that have broken their taboos willingly, or have been forced to, and most of them end tragedically.) It is believed, that a breach of the taboo, even if kept secret, will sooner or later turn on the offender.
The thing is (what they believe), people _have_ to believe in something, they must have something pure and stable in their life. Otherwise, they cannot be really considered more than animals. Even that cutthroat knows to be in some way better than criminals in other towns, because he keeps to his word, and never lies. While many of the people living outside are surely honest, their life must be a mess.
The taboos invariably result in the separation of the society. Honest people get along with honest people, liars with liars (somewhat). Of course, liars and thieves have practically no chance to reach certain positions, like judges, or clerks in general. From a point of view, the society is divided into castes of sorts; but movement between them is possible.
If people learn to know others, they often talk in detail about the taboos they have acquired, when and for what reasons. Some tend to boast about the moments they were tempted to break a taboo, but resisted, others consider it indecent. People with broken or abandoned taboos, even if evaded, draw the attention, and some are very eager to hear these life stories.