No one just starts giggling and wearing black and signs up to become a villainous monster. How the hell do you think it happens? It happens to people. Just people. They make questionable choices, for what might be very good reasons. They make choice after choice, and none of them is slaughtering roomfuls of saints, or murdering hundreds of baby seals, or rubber-room irrational. But it adds up. And then one day they look around and realized that they're so far over the line that they can't remember where it was.