Twenty children and six adults killed in a town in Connecticut. Why? As someone who listens to killers as an expert psychological witness in murder cases, I have spent much of the last 20 years trying to understand how and why young men kill, maim and attack others.

Killings like those in Newtown, Connecticut; Aurora, Colorado; and Virginia Tech are always met with expressions of shock, anger and sadness. These are understandable first reactions, but in the long run they accomplish nothing.

So long as the discussion does not move beyond labeling these events "senseless violence," horrors such as these never move us closer to a place of deeper understanding. Greater understanding is crucial because understanding leads to more peace and less violence through preventive action. All the crime scene investigations in the world will not do this.

Although all our instincts urge us to dissociate from the killer, achieving better understanding requires us to put ourselves in his shoes no matter how frightening and distasteful that may be. I have done this over the past 20 years, and I have learned that it's the only way we can understand a fundamental truth: Although to the rest of us, the observers and the victims, extreme acts of violence seem "senseless," these murderous acts make sense to the shooters.

This is true whether it's Adam Lanza in Newtown, Connecticut; James Holmes in Aurora, Colorado; Seung-Hui at Virginia Tech; Dylan Klebold and Eric Harris in Columbine, Colorado, and the many thousands of others who wage war against their society, either in the form of high-profile massacres or the daily grind of shootings around the country that barely make the local news.

How do we go about this process of "making sense," not as a way of excusing but as a path to understanding and preventing violence? We start by recognizing that many young Americans (and other young people around the world) develop and carry with them a kind of moral damage, which I have come to call "the war zone mentality."

However it develops, they grow up with a damaged sense of reality. They view the world as if they are soldiers confronting a hostile environment that they perceive to be full of enemies. Once they get fixated on this damaged world view, they may hatch the delusion that even teachers and young children are their enemies. For Adam Lanza, apparently even his mother was an enemy who had to be destroyed.

There is no one cause. It is as if they are building a tower of blocks, one by one, that can get so high it falls over, with innocent people dying. These building blocks can be found in a dangerous neighborhood or a school rife with bullying. They can be found through the Internet and mass media: the many, many web sites and videos that promote paranoid views of the world and validate violent action in retaliation.

They can be found in pervasive and intense playing of video games, the hands-on virtual violence that desensitizes young people to proxy killing. These games become a psychological pathway to real killing by dampening impulses of compassion and altruism.

They also come from a culture that supports access to lethal weapons: the crazy availability of guns like the Bushmaster semi-automatic rifle used by Adam Lanza that are, in effect, weapons of mass destruction when turned against children at school, or moviegoers in a theater or shoppers at a mall. These weapons have no place in civilian life.