Why Is It So Hard to Be Evil in Video Games?

I have always loved the idea of choosing my own path in a game. Moral dilemmas make virtual worlds more interesting. Sometimes they change the outcome and give you a reason to play the game all over again. But as much as I like the idea, I often struggle to take the evil route. I'll replay a game with the intention of being bad, yet I find myself being a Good Samaritan again.