Everything from bad parenting to violent video games has been proposed and there's been a certain amount of attention on the role of movies, particularly the influence of The Matrix. With its violent content, special effects, and most particularly the trench coats worn by the lead characters, some analysts saw a direct connection between the shootings and the movie. There's a facile assumption amongst pop psychologists that movies are more violent than any form of entertainment ever seen before by human beings.
Violence is part of the human condition, an evil that lies beneath the veneer of civilization.
What is fascinating about The Matrix is not its use of guns or its content of violence, but rather the premise that underlies the movie. In reality, it is 200 years later and the world has been laid waste and taken over by advanced artificial intelligence machines. It is a vision of the future that The Matrix captures best - an anxiety that many young people speak of when they discuss their lives in the 21st century. When Morpheus shows Neo the world in 2199, the earth is a burned out shadow of its present self, destroyed by ecological disaster and human warfare, with people huddling in caves, struggling to survive. It is a future where the artificial intelligence created by the computers is more powerful than any human intelligence or emotion.