I rather enthusiastically entertain the notion of a game that lets you play as an evil character. The concept of strolling into your favorite RPG village and laying waste to all those familiar yokels is appealing. Playing a power hungry megalomaniac also has a nice ring to it. However, when playing an evil character your acts become banal when you've slaughtered the same village 10 times, or chopped the head off of every passer-by, or even gone on several rampages against the authorities(i.e. police, guards, etc...)
For an act of atrocity to truly feel atrocious, it has to be being observed from the perspective of someone who considers themselves good. They also need to see the one committing the act as good prior to the act.
If you go along robbing and killing everyone you meet, everyone will expect that behavior and in time come to accept their plight as the oppressed.
So how do you create a world for an evil character, where the acts of evil mean something? How can an evil character be seen as anything but evil? And how do you create things for an evil character to do that don't become boring after the first couple times the player does it?
The game industry has yet to satisfy these questions with any current games. Maybe you or I can find them.