Genetics and violence

A recent article describes how some people (with a specific gene variant called "MAO-A") have a stronger reaction when "shown pictures of angry or fearful faces". 

"MAO-A breaks down ... neurotransmitters [in the brain] so that they no longer exert their effects on the nervous system".

"Researchers showed that males with lower levels were more likely to develop antisocial behavior in response to being maltreated during childhood [perhaps] because they are less able to reduce their neurotransmitter levels ... after a stressful event, limiting their ability to control aggressive impulses." 

In other words, people will react differently to the same environment, due to variations in their genes.