Zaeed's mission in Mass Effect 2: at one point you need to choose between chasing down Zaeed's old nemesis and murdering him, or rescuing a group of colonists trapped in a burning building. On the one hand, if you help Zaeed kill his nemesis before he gets away, you get his loyalty - potentially giving you an edge in the final mission - and an achievement. That's an in-game reward and a meta-game reward. If you save the colonists (and it was Zaeed's fault they were even in danger adding an element of responsibility), you get jack shit, save a few Paragon points.
I sat there staring at the dialogue wheel for like ten minutes. Finally I decided I couldn't reconcile abandoning the colonists. Zaeed nearly killed me over it, but at least my Shepard could look himself in the mirror.
Oh ho, Dragon Age, though. I played that game as an evil mage with a single goal: gather as much power as possible, no matter who I had to fuck over, backstab, murder, leave to die, trap forever in a state of limbo, or curse to possession by demons in order to do so. If the game had let me usurp the throne, I'd have done it in a heartbeat.
That game does a really good job rewarding you for being ruthless. I cut a fiery swath through Ferelden.
Being evil is easy in fantasy games because I feel compelled to punish everyone for their horrendous faux-British accents. My girlfriend got me to try Fable 2 briefly, and it infuriated her that every time a character started talking to me, I would ask her, "Am I allowed to kill that person?"
Never had trouble with Little Sisters, either. I found them overacted and unconvincing as little girls, with all their loud proclamations of "Mister Bubbles!" looping way too often. It never feel like I was doing something evil to a person - it simply felt like I was taking a cartoonishly unsubtle 'evil option' in a videogame.
It really comes down to how believably the characters of a world are presented.