The villain of my now submitted novel is Jim Jones of Jonestown fame. This is the man responsible for the deaths of 900 people, including children.
If you read his story you'll know that he started out as a kind, loving, caring man, a preacher who was one of the very first to allow black people into his churches and who, when he was forbidden to do so by his higher-ups, chose to give up his job and start his own church. And who fought for the rights of blacks, old people, disabled, and had a dream of a better world where everyone was loved and respected.
Even at the end, he was convinced that his "mass revolutionary suicide" would open the eyes of the world to the fact that he was a saviour.
I think making him a good villain involves showing him from all sides - developing some insight into his psyche so the reader can see how the change came about. There was method to his madness, and I wanted to show him as three-dimensional, not just as a paper-cut-out evil monster.