Murderers are born and not made. Wrong brain wiring due to non
existent god , or hit on the head as  a child and brain damage.
   

A man who murdered two young women and tried to kill a third has been
given a "whole life" sentence.
Levi Bellfield, 39, from west London, bludgeoned Amelie Delagrange,
22, and Marsha McDonnell, 19, after they got off buses in south-west
London.

He was also found guilty of trying to kill Kate Sheedy, then 18, in
2004 and has been named as a prime suspect in the 2002 murder of Milly
Dowler, 13

excerpt
http://psychologytoday.com/articles/pto-19950101-000002.html

Natural born killers?
Questions whether violent criminal behavior is a mental disorder.
Research of Adrian Raine, Ph.D., who discovered that the prefrontal
cortex, the brain region behind the forehead, was less active in
murderers; Further details of the study; Implications for justice--and
for rehabilitation.

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BRAINPrefrontal Cortex

Is violent criminal behavior a mental disorder? A University of
Southern California psychologist says yes--and he'll show you brain
scans to back up his claim.

Adrian Raine, Ph.D., led a team that compared brain activity in 22
murderers and 22 normal folks. Their tool of choice: the PET scan, an
imaging technique that measures the brain's utilization of glucose,
its primary fuel. The scans indicate which areas of the brain are
active--and which are lying low.

The researchers discovered that the prefrontal cortex, the brain
region right behind the forehead, was less active in the murderers.
Prefrontal deficiencies have been associated with a variety of
behaviors--risk taking, rule breaking, aggression, and
impulsivity--that can lead to violence.

But there's a catch. The murderers in the study had all pleaded not
guilty by reason of insanity. Might mental illness account for their
abnormal PET scans? No, says Raine; insanity is a legal concept, not a
medical condition. The variety of mental disorders the killers cited
in their insanity pleas do not explain their lower prefrontal
activity.

If further studies confirm that murderers' brains are biologically
different, does this mean that some of us are natural born killers?
Cognitive remediation training has helped brain-injured patients
recover lost function. If such therapy is able to help violent
offenders beef up their brain to compensate for an underactive
prefrontal cortex, the changes might show up on a PET scan. Come
parole time, those scans could be far more convincing evidence of
rehabilitation than a convict's professed remorse.