Good vs Evil tends to not work in RPGs for me. The issue is, that good paths aren't selfless - I think it comes from a mechanical need to give player incentives and rewards. You might help tieflings because you are selfless - but if that fails, Halsin is your best bet at saving yourself. And you get promised loot for helping them. And in the process you make bunch of people like you. No matter who you play as, you are given motivation (both for your character and you as a player) to follow the good path.
Ah, the old 'do true selfless acts really exist?' question. I lean towards a 'no', because there is a rationality behind being good that has to do with building up a society or a life that works. Ideals of being good are not just random ideas, like 'I want to help people just because..', even in religion there are reasons behind the laws you must follow. So when you help someone you may not get a reward right away, but the people you helped will remember, and maybe they'll pass on the story and pave the road for you making others more willing to help you when you are in need. Does that make your act less good? I don't think so. True selflessness is an insane ideal, you cannot help anyone if you don't take care of your own needs too. So about the game rewarding good acts (as well as selfish ones) I think is quite alright. It does not have to be in the form of material goods though, can be more subtle. And yes, there should also be room for just being good or helpful without getting anything in return, except perhaps inner satisfaction.
But that was a sidestep explaining something about 'good'. Original question was about 'evil'. Many of you have given nice explanations of what motivates you to be evil. Enlightening, but also a bit disturbing
