Oh man, so much wrong here. I don't want to sound like I am running someone down. But you need to look at other things.
But there is one significant difference between me at 16 and 17 years of age and most high school shooters: I didn’t play violent video games.
As a child, my mother taught me that all video games were “evil.” That’s the word she used. And although that word might be a little extreme, I grew up thinking that there was something very, very wrong with pretending on a video screen.
And because he never played video games he also knows nothing about them. Sorry guys, even 3 out of 4 is way too low a number. You have to take it into the 90% realm. Given numbers like these you are talking a probability so low that you can't even argue a minor influencing effect.
The Army didn't change the shape of targets to desensitize soldiers to the idea of killing. They changed the shape to help train target recognition, to rapidly identify the shape of a human at distances in a varied setting. And because a hit or a miss is all they cared or had time to worry about. Bulls-eyes become immaterial for the average soldier shooting open sights at 200 Meter+ distances.
Here is food for thought.
Take a kid at age 1, put him in day care where no one really cares about him, just about making money off his folks.
At age 5 or 6, put him in Kindergarten or preschool or whatever they call it. same thing again.
Along comes the professional Teachers in Grade School and these kids are still trying to learn how to get along with each other. Schools don't teach kids how to handle aggression, there is no outlet, You have classes full of undisciplined kids and they can't "take it out on each other" without getting into trouble. But much worse is that they can't defend themselves either. They get two ideas driven into their heads, I can be beat on, or I can do the beating, either way I am getting into trouble but the trouble is either being sent home, (not so bad to the immature), or being labeled bad boy. But there is no real punishment that compares to the actual issues they face dealing with each other. This is real, this is every day for these kids. This has a much greater impact on their development and mental health then a video game.
Now if you want to demonize a type video game, demonize MMORPGs like Warcraft where kids are almost trained and indoctrinated that being the good guy and playing by the rules are not the best ways to win. That there are no punishments for immoral behavior, theft, cheating, rudeness. But they do get to abuse other people online, they do get to get that out of their systems. They get to do it in all social avenues online, facebook, twitter, and within MMO type video games.
Somewhere along the line throw in a divorce and a little home violence because two working parents are just two very stressed people so even some yelling and throwing of the dishes is a common thing.
Then add a little rytelin or whatever drugs and a world where kids are told college is coming, work is coming, no one is going to hire you, girls don't like you.
Now just why on earth would I have a rosy outlook on life after the first 16 years?
The biggest single reason these kids are having these issues is cause their mothers are not home raising them and their fathers are not involved, and their mothers and fathers are getting divorced or never were married to begin with.
That's my take.