There was a researcher who was trying to predict what makes someone a serial killer. He interviewed and ran brain scans on numerous serial killers and found three overlapping factors:
1) An injury to the forehead--this is where the prefrontal cortex is, which is associated with our conscience (70% of serial killers have received some type of head injury in childhood/adolescence)
2) Experiencing abuse as a child (physical, sexual, etc.).
3) Mental illness
Of course many factors play into the making of a serial killer, but this is what his analysis showed.