I also prefere it when races can't be truly defined as evil. Some races and cultures may be more suited for evil then others, but I think that all should have some option to be both.
You could maybe have a fallen faction that instead of killing the weaker, transforms them in something stronger. That faction could claim that it wants all people to become strong and powerfull, it could pretent to be the savour of the weaker men. Yet could that "transformation" involved magic or technology that makes it a rather unpleasant process and some could view that forced transformation as losing their self.
Having factions that are not clearly good or evil, but who have elements of both, adds to the game I think. This uncertainity adds to the story and the drama of the world, it makes choices real choices(because you are quite never truly sure whatever you did the good thing)