The Discovermag website has an interesting article titled:
Into the Mind of a Psychopath
The article discusses the life long challenge which Dr. Robert Hare had set out for himself. That being trying to understand Psychopaths.
Psychopaths and Narcissists share a number of key traits. Both lack empathy for others. Both can be extraordinarily charming The charm is used to get the victim to lower their natural fight or flight defenses. The charm reels the victim in for Narcissistic Supply or Sadistic Pleasure. Both have no remorse for what they do to their victims.
The article is a good read about one of the pioneers into the research on Psychopaths. Robert Hare was the first to use MRI Imaging to show brain scans of Psychopaths.