The most notorious killers -- Bundy, Gacy, Ridgway, Sells -- have claimed hundreds of victims and share one thing in common. They are all psychopaths. It is estimated that one in every 100 people is a psychopath. But what pushed these psychopaths into serial murder? Was it nature or nurture? To understand their savage brutality, we will travel back into their childhoods, reveal startling new scientific research about psychopaths and meet one of these killers face to face.