Games-with-good/bad-character-systems-playing-menz

How do you play an evil character? I've played all manner of games which offer the choice, but I can't say that I've ever played an evil character unless it was forced on me. And it's not for lack of wanting to really, though there is a little bit of that--I try to play these sorts of games as myself as much as possible, and I really am not an evil person; I'm an asshole sometimes, but not evil.

But a lot of seems to stem from not wanting to piss off the characters I actually enjoy being around, not wanting to accidentally kill a random civilian who might prove useful later, and not wanting to shortchange the game's story.

So how do I do it? How do I play a so-called evil character without breaking the game I'm playing? What are some games that really reward you being evil beyond just the personal grin you get from being a dick (i.e. a better story, etc)?