How does someone grow up to be a serial killer? Is it genetics?

How does one raise a child to become a serial killer? I've been reading articles about the most evil of crimes, and I cannot believe how a human being can turn into such evil. I was wondering, is it genetics? Or is it something that the child develops based on how he was raised? How does someone grow up to be a serial killer? I mean not someone who just got drunk and angry and killed someone by accident. I'm talking about someone who goes on a murder spree in the span of years. Is it something you were born with, encoded deeply into one's DNA? Or is it the environment that the person grew up in?