Like many things I do, it's partly escapism. Reality can be a tough pill to swallow, and I lost my idealistic fire about the time I (1) fell in love and (2) realized that I didn't have the will to try to depopulate the planet.
I bet a lot of you think I'm joking about that last. No. Really. Falling in love does a lot to get a fellow to stop wishing death on his own species. It does not, however, eliminate the problems that species causes or that same fellow's awareness of those problems, hence the escapism.
Partly, it's an exploration of my own limits-- though after nearly three decades, I consider those limits pretty well explored, so I don't really, consciously play "evil" characters anymore. Basic sketch of results: I'm capable of immense impersonal destruction, but seem to have real trouble with knowingly being a jerk, even if it's in character.
Partly, it's to play around with ideas. Aria's always been a little bit of a philosophical cat's paw, a way of exploring principles and ideas that I'm not quite ready to commit to, personally, often including dark corners I'd never seriously consider wandering around myself.
Aria kills people. I just defend them in courts of law. We may have similar philosophical bases for why, though.