The July issue of The Writer features an interview with novelist Diana Gabaldon ("Dragonfly in Amber," "Drums of Autumn"). She was asked why writers often don't get villains right. Part of her reply:
"First, there is a squeamishness that leads people to be superficial and stereotypical about their villains because they don't want to go deeply into them. Yet, they want them to be very bad, so they're picking out surface aspects, which means it'll never be convincing. ...You do have to accept them as actual human beings and also as parts of yourself, which is where the emotional honesty comes in."
By the way, that issue also features one of my "Write it Right!" cartoons.
(Great dialogue comes from great characters, and that's a topic covered in depth in my book, "Your Writing Coach," published in the US and UK by Nicholas Brealey and available from Amazon, Barnes and Noble, and bookshops.)