I think one of the best explanations of this can be found in an old episode of the Transformers: Beast Wars cartoon (yeah I know; NERD!). This is an excerpt from one of the many long speeches given by the Evil Megatron:
"That's the problem with heroes, really. Their only purpose in life is to thwart others. They make no plans, develop no strategies. They react instead of act. Without villains, heroes would stagnate. Without heroes, villains would be running the world. Heroes have morals. Villains have work ethic."
That speech put a new perspective on the evil mastermind thing for me. It might also explain why I seem to enjoy writing the villain parts more than the heroes. There are many lessons to be learned from the stories of our childhood.