Play both good and evil, but I never get so evil that I'm out slaughtering entire towns or anything. More of a sinisterly evil.
More often then not, and for my own personal amusemant(roleplaying) my character will evolve as the game progresses. My good character will become less heroic and eventually sway from his altruistic, moral nature; indulging his darker desires and evil tendencies. And my evil character will find redemption along the way, until finally committing himself to selflessly helping all those in need as penance for his life of sin. I usually try and work the transformation into the overall story/quests in some way that makes sense.
TOO MUCH--maybe--but I like it.