Are you capable of Murder?

I hope that everyone has read their newspaper today and saw the cover story about the city council shooting. Unfortunately, Two police officers and three city officials died after a Kirkwood man took murderous action on his frustration toward city officials. After reading the story, my best friend Melissa became unusually concerned. "He was the nicest man" was her response. 

It turned out that she was very familiar with the shooter in Kirkwood. She grew up in the Kirkwood area, and  they lived in the same neighborhood. She said that he was always friendly and gave Melissa and her sister candy wen he saw them. This lead me to think about various other murderers, and the news coverage of their neighbors, relatives or close friends saying how shocked were. So my question is: What makes a murderer? Does it take a crazy person to take someone's life, or is anyone capable of loosing control and committing the act? Considering the how often shootings take place in America and my life experiences, I find it more and more difficult to find the answer.