In most RPGs, I usually like to play characters of questionable morality, or characters that contrast with their alignment, assuming there is one. For example, in Star Wars: The Old Republic, I played two characters, a Jedi who was becoming increasingly seduced by the Dark Side, and a Sith who might've been a Jedi infiltrator, sabotaging the Empire at every turn.
I don't care for heroics, or unjustified villainy. I like to mix it up, and try to carve as interesting a character as I can, and I don't see the point in just playing an avatar of myself.