...For those who feel that evil, by the very basic nature of it, cannot work together: I present the book 'Villains by Necessity', as well as the note of Thieve's Guilds, Hitler, Napoleon, the note that 'evil empires' remain a staple of fantasy everywhere, and that evil is nothing more than the desire to get ahead, without the considerations for the [n]good[/b] of others that stop other characters. Many corporate sorts, interested only in climbing the ladder after the elusive carrot of a higher income, could be considered evil. In all honesty, I myself might be considered evil, by many people - I don't really care about the people who I don't consider my family.
This doesn't mean I, or others who mighty classify as 'evil' (such a thing only truly exists as a concept, anyway - good and evil are both ways to justify what you do and why the other guy needs to do things your way)go out randomly slaughtering people and backstabbing out partners. A utterly unreptentant psychotic 'Chaotic Evil' might - but few people fall into that extreme. Neutral Evil, in D&D terms, would be best called extreme selfishness. And lawful evil - hey, follow the rules. Just figure out how to use them without breaking them. And that's only if you're serious about it. A lot of evil types might be perfectly content with their lives, as long as it isn't disturbed.
Evil people can have intensely strong friendships. Love, joy, camraderie - those aren't exclusive to Good Guys, after all. Neither are hate, fear, and rage Bad Guy only emotions.
Killing a helpless foe is evil - but perfectly logical, to ensure it doesn't heal and come after you later.
Evil campaigns can happen just as surely as good ones. It's simply that very few people have even the slightest clue as to how an evil character would even act, and therefore often play it in the twinkish fashion that most people link to 'evil' players.