I can't. There're two reasons for this, and neither of them are "It's just wrong":
1) The black/white-feeling how it's done. Like, bad guys are always child-eating, puppy-smashing, granny-kicking assholes who spit on everyone's face and always get negative reaction from everywhere. I just can't play with that kind of character, not without being sick of my own evilness at least. Hell, I can't even understand why they'd ever be interested in avenging their father or stuff like that, which is what you usually do in RPGs.
Now, if there ever was a possibility to play with a greedy, cheating, manipulating bastard who swindles people from their money without them even noticing, does good deeds just for the reward (while our generic bad guy wouldn't do it at all), and generally seems a nice enough guy to other people, like a nice red apple that's just rotten from the inside - that'd be a different story altogether. I'd love to play with someone like that. Too bad it's just not possible.
2) Good guys always get, not only the nice feeling of doing something that matters (that should be enough for any good guy), but also much much bigger rewards and loads more experience than the evil ones. Maybe it's just being such a generic asshole they always are, but true baddies should get more money than shiny knights by default. It's just how it should go.
Like, let's say you're supposed to deliver some money from one guy to another. The good guy does how he was asked, sure thing. The Generic Bad Guy takes the money for himself, and goes to the NPC to brag about "Ha ha, I'm gonna keep the money myself, and there's nothing you can do about it! Ha ha!", and something negative, such as bad rep and everyone hating you, follows. Meanwhile, all he really gets is just some meaningless little sum of that you can't buy anything useful with, and you'll get so much more money by selling the stuff from any random encounter (known as "Fallout 2 Syndrome").
Meanwhile, the good guy who did as was asked, will get to keep the whole sum for himself for being such a nice guy, and the NPC will give him some Really Powerful Magic Artifact That's Been In Our Family For Generations (tm), and everyone in the game loves the player a bit more.
A baddie I'd like to play would grab some of the money for himself, and then deliver the rest as asked. With luck, the NPC won't notice that some gold is missing.
So if you want the most experience and the best stuff, you'll play with the Innocent Momma's Boy. I want my games for entertainment and escapism, not ethics education, thank you very much.