It's one thing to go head-to-head with an Evil being. And still another to play an Evil character. But what happens when you're minding your own damn business and someone else in your troupe decides they don't want to be Mr. Nice Guy? Now, that's a real dilemma.
Probably one of the biggest difficulties is that being forced to play with/hang out with an Evil character TOTALLY doesn't mirror real life at all. Someone treats you badly in real life -- you know, steals your money, slices your brake lines, sticks a knife in your back -- you get the hell away from them! You can't exactly do that in a gaming group. I mean, I suppose you could, but who wants to roleplay a bunch of solitary characters that have little interaction because they can't share a sandbox? Yeah, no fun. So, you find yourself in a situation where you have to play with someone that, under normal circumstances, you wouldn't let within spitting distance.
Another real challenge has to do with the separation of "player knowledge" from "character knowledge". Do you KNOW that their alignment is Evil? If you don't, then no worries. You'll figure it out and it'll happen in character, and you will be justified in what you have to do to them (that is, if you don't join their cause). But if you do happen to know they're Evil-aligned, well... hello! Time to see just how good a roleplayer you are. Can you leave what's written on paper alone and separate yourself and your knowledge from your character's knowledge? Harder than it sounds. And really, I prefer for players not to know each other's ethical center. That's not something you get in real life. We don't run around with ethical "mood rings" on letting each other know if we're a right bastard or not. But sometimes it happens. And it's hard as hell to leave that knowledge out of play once you know it. See it all the time... suddenly everything that player does is suspicious, though it was fine not half an hour before. Yeah, real challenge. Better you just don't know.
Then there's the challenge of working someone with ulterior motives into an overall greater scheme, the residual doubt in what s/he has to say or do, the heightened tension wondering if the knife's going to come from the back or the front... it can be downright harrowing. And exciting!
Because Evil characters provide tension and extremism, they also offer the opportunity for experiential character development for the other players in the troupe. Not all experience is in points and money. Character experience -- what shapes and defines who your character is and how they go about making future choices -- has to come from successes and failures in decisions and relationships. And truth be told, there are few things that will change a character's fabric faster than dealing with someone not nice. One encounter can wither an idealist, birth a cynic, and give a realist concrete shoes.
Have you ever played with an Evil character in your troupe? What kind of experiences did you have? Did you know their alignment while you played? How'd you feel when you found out?
Or... did you try to save them? Cause that never works.