That's something that I studied about in one of my classes last year (Psychology), and it fascinated me.
Last night, I watched a documentary on TV about psychopaths. The background: This researcher decided to look for a common theme among psychopaths and why they kill so mercilessly. And he DID land on something- he did find characteristics (physical & genetic) that all psychopaths have. So it was really interesting that they found out why psychopaths have no emotions whatsoever.
Then the documentary took an interesting turn- the researcher was informed by his mother that one of his cousins' family line had 16 murders. Intrigued, he tested all his family members and aloha! Of all people, he displayed the characteristics of a psychopath- but of course, he's not a killer.
So he had to look for more answers- what causes psychopathy to manifest?
Here's the first part of the documentary, I highly recommend checking it out!