On Evil
I believe, aside from a few psychopaths, "becoming evil" is not a choice, in the sense that reasonable human beings do not choose to inflict pain and hurt without thinking they are also causing good and order. Everybody is a hero in their own story.
Mary Shelley, Frankenstein
No man chooses evil because it is evil; he only mistakes it for happiness, the good he seeks
C.S. Lewis, The Humanitarian Theory of Punishment
Of all tyrannies, a tyranny exercised for the good of its victims may be the most oppressive. It may be better to live under robber barons than omnipotent moral busybodies. The robber baronβs cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end, for they do so with the approval of their own conscience.
Jim Butcher, Cold Days
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.