Being a citizen, as I am, of the U.S., my view is that there's no singular, let alone simple, answer to the question of why certain people "go off the deep end"...
Look, the last 3 mass shootings that occurred in schools, (Columbine, Colorado, Red Lake Minnesota and now Newtown, Connecticut) were perpetrated by young, white males. In fact the history of the last 30 years or so only includes one such shooting that involved a female. Surely testosterone has some affect on the incidents.
AB Editors question asked if video games had/have an affect. After watching some of the games in progress at a game store recently(my first exposure) I can't help but believe the detailed blood, guts and gore that are displayed when the game's operator pulls the trigger have a numbing effect on that kid. It's unreal, in his mind, in my opinion. He's able to kill (and be killed) and yet his victims and himself are 'resurrected'. For the person who has other personality problems, this must divorce his reality from actual life and perhaps he doesn't really expect those he has murdered to 'stay' dead.
It's significant, that most of those who do the killing usually kill themselves, leaving us to try and determine, post-mortem, the reasons.
Some have commented on the numbers of guns in the U.S. I own 14 long guns, shotguns and pistols my self. But most of those, like friends I know, collect them for the value or uniqueness (2 are black powder muzzle loaders). I've hunted (living as I do in the western U.S.) all of my life and was taught early (age 6) by my father and grandfather how to handle and respect the firearm safely. That's not a singular experience among those I know who own guns.
Some collectors have guns running into the hundreds. But, in fact, of all the people I can think of, probably less that 30% own a firearm.
I also have (as do most of my friends) a permit called a Conceal-Carry License. It allows, as the name suggests, the carrying of a weapon in a concealed fashion... and I do. Never had to use it and don't expect to, but I'd rather have it and not need it than not have it and watch my family (or others) be harmed. But I recognize that attitude is part of my upbringing as well as self-reliance imposed by living in the unpopulated western U.S.
At any rate, the statistics (what's the old saying about statistics being manipulated by those who have an agenda) seem to indicate little, if any correlation to owning guns and murders committed by them as tools. The last horrific murders at Newtown were done by a young man who was deeply troubled, spent long hours in a darkened basement of his mother's house playing combat video games and used his mother's legally obtained guns to murder her first and then the children at a school.
A prohibition was imposed in about 1992 by the Clinton Administration on owning the so-called 'assault weapons'. The murder rate by using such weapons actually increased... significantly increased. The law was repealed in 2002 and the murder rate using them went down 52%!
Keep in mind, the ownership of true automatic weapons (pull the trigger and hold it and it keeps firing) has been illegal for 50 years. A semi-automatic type is one that fires with each seperate pull of the trigger, which are legal.
I don't have an answer for how to prohibit this in the future... I just know that confiscation of all the guns the police force can find will only leave illegally obtained guns in the hands of such individuals as the school murderer.
By the way, being armed to the teeth preparing for Armageddon is a position only held by a very small group of folks whom we even deride, since they haven't clue as to what life would be like in a full blown social system collapse.
Many of us were fortunate enough to have parents or grandparents who lived through the Great Depression and truly knew what near collapse was and taught their descendants thier own hard earned self-reliance...