#7 Jean-Claude Romand
The French killer posed as a doctor for 18 years. He was so convincing, that Romand fooled his family and friends, who all thought he was a successful medical professional and researcher in the World Health Organization. He convinced people he had conducted major research and had close ties with political figures.
He took extreme and deadly measures to protect his secret. He murdered his ex-mistress, wife, children, and his parents.