Let me start with the good about this:
I think giving troubled youth self-sufficient exercises may be good for them. Not to mention, I live in Florida of all places and at 17 I suffer from a Vitamin D deficiency. This is caused by lack of sunlight and can cause depression and mood swings. Getting outside (to the range, almost ironically) makes me feel better.
That said, video games training for you to kill people? Have you ever actually played a video game? I've played them since I was 4 and I will guarantee you they've done nothing to teach me how to kill people. The extent of using the guns in those games includes holding one button to bring up the sights, using the right stick to aim, pressing the trigger to fire, and then pressing the square or x button to initiate the reload sequence.
Not exactly military style training.
In fact, I would argue that a young kid who has taken 30 or 40 deer in their life is MUCH more trained than a person who spends even 80 hours a week playing video games. They understand anatomy to some degree to get kill shots, and know how to get hits on a living being and handle a gun.
Do I think we should stop people from hunting? No.
As far as being mentally prepared, I don't think people talking about these troubled teens quite understand them. You know what will prepare you much better? Snuff videos. Videos of people hanging, being shot, stabbed to death, beat to death, eaten by animals. These videos have been available since the 60's and today I could probably find one in less than a minute. I've studied them as I'm fascinated with the history and legal grounds but have never had the fortitude or even desire to watch them. But I guarantee these videos are available and much better for that than video games.
Not to mention, how many kids do you think play these games? I'm going to say 2 out of every 3 or 3 out of every 4, at least. Boys, girls, from the age of 5 or so. No wonder all these kids at the shootings play these games. That is literally as bad as saying we should ban AR-15's because they're present in 4 out of 5 mass murders. It just happens to be so because they're the most available, and probably because they're more comfortable than competitors at the price.
I don't understand how gun-rights supporters can say "Don't even consider maybe looking at if our guns are related" but then say "It's the video games! Don't let kids play those!". I agree we should have no gun restrictions (I've previously stated I'm for ownership of just about anything non-nuclear that's not covered by the Geneva convention) but the right to play these games is a liberty too. Not to mention the evidence supporting this being the cause are just as shaky as saying guns are the cause.
Video games don't kill people, people kill people.