I love bad guys, especially when, at first, I don't know how bad they are.
Maybe I like a touch of mystery, or maybe I'm just one of those people who was born to learn not all people are good. My husband can spot a hustler a mile away. I've needed an empty wallet or bruise marks.
I character sketch my villains as thoroughly as any hero and heroine. He needs to be a worthy opponent who makes a happy life impossible for my main characters. Sidetrack the hero. Nearly seduce--or kill-- my heroine. Trigger emotional responses in me, and a reader.
Until he is completely and thoroughly exposed, ruined, and destroyed.
Justice, good, and love triumphs over evil.
Happily Ever After.