Evil is sexy...not to mention laughably easy to play in any game with a moral choice system. For instance, why would I bother running around Fable 2's boring and repetitive good-aligned quests, when to be evil all I need to do is stand in the center of the first city and spam lightning? Why, in Fallout 3, would I run around the overworld at a snails pace to do the research I promised to do for Moira, when I can leave her store, turn around, walk back in, and straight-up lie to her?
Luckily in a lot of actual RPGs this is avoided. Evil is as hard to play as good in most of the Bioware games I play, for instance, and most games based on DnD add more depth to evil choices, and raise some moral dilemmas in the process (ie, and this is a very rudimentary example, is it okay to rob and kill people who are themselves thieves and murderers?).
Most of the time I'll choose evil when it's easier, but only because what games count as being on the up-and-up is tedious, irritating, and just plain annoying.