Wendy McElroy describes how evil is wrought in many tiny steps:

I was once asked to describe the devil. (I interpreted the question to be about the general nature of evil in man rather than about religion.)

I replied: If the devil is the living flesh of evil, then here is who I think he is. Far from appearing as a hideous demon, he is the average-looking person who walks into a room and shakes your hand with a smile. By the time he leaves, the standards of decency of everyone within that room have been lowered ever so slightly.

Perhaps he offers general statistics on divorce or child abuse to convince you to suspect your husband of infidelity or your neighbor of molestation. No evidence of specific wrongdoing is offered, of course. But since such "crimes" do occur, you are advised to be vigilantly on guard against them in your personal life. And so, you begin to view your spouse and neighbors with a bit more suspicion, a little less trust and with the tendency to interpret every action as possible evidence of wrongdoing. The very possibility of an offense is taken as evidence of its presence.

Perhaps he spins a political theory that inches you toward viewing people, not as individuals to be judged on the basis of their merits, but as members of a class. And so, your co-worker is no longer an individual; he becomes "black" or "male" or "gay" and his actions are interpreted according to his category.

Slowly, you come to view the world through the eyes of the devil.

RTWT. (Hat tip: Intergalactic Capitalist)