For the record, I don't blame metal/video games/pot for these shooting (although stamps could be at fault). Nor do I blame the existence or proximity of guns.
The way I see it, parents who own such lethal weapons have to be aware of the mental state of their children. If there's any risk at all that the kids might try something with a gun, whether due to anger, depression, ignorance, or anything else, then it has to be kept under lock and key. It's as simple as that. And don't give me any of the "Oh, but they're too busy!" bullshit. If that's true, then they can assume the aforementioned risk is there and lock up the gun.
Also, please take note that I don't own a gun, none of my friends or family (to my knowledge) own a gun, and the only time I ever fired one was at a target on vacation - I never hit the target, nor the piece of paper on which it was printed. I've long since concluded that I'd stand a far better chance in a fight without firearms... but I have no problem with them. I have a problem with some of the idiots who own them. If only background checks could legally include stupid screenings.