Still reading Reflections on the Magic of Writing, by Diana Wynne Jones, and came across this advice to young writers, the vice versa to protagonists having a shadow side:

“You have to remember that villains are real people too. They have reasons for what they do . . . and they do not, as a rule, regard themselves as evil. They are acting for a cause, or out of deeply held convictions which have led them the wrong way. A lot of writers forget this. They make the baddie give evil laughs and rejoice in his/her wickedness — or worse, they wriggle out by making the villain mad. . . .The majority of bad people are not like this.

“And here is a tip, something I often do. Make your baddie someone you know and dislike. Use a real live person. Then there will be no trouble in making him/her convincing. You know them anyway. People are often shocked when I say this. But, since no bad person ever thinks of themselves as bad, these live people will always fail to recognize themselves and there is no harm done. Besides, they deserve it.”