This is something I've noticed doctors have been trying to point out, something pushed these guys over the edge and the mixture of being hopped up on drugs and not being treated right would do that to anyone nevermind people who have mental problems to begin with. Good luck ever getting anyone in America to admit that though, they conveniently ignore the fact that Canada and Switzerland have just about as many guns as the Americans do but you very rarely if at all hear about people going on a rampage with them.
In fact, minus the compulsory national service, I think that the Swiss way is the way to go, if you want to go and buy a gun or have a rifle, you've got to do some kind of proper training first, these 'accidents' I've seen people list as a blame against firearms are caused by people doing things I would never do with a firearm and I don't even own one or know how to take it apart properly!
That is a pretty enlightened view point from someone I would traditionally disagree with. As a libertarian I am pro gun but I can also see the value of proper instruction in the use and storage of firearms. I would not advocate mandating it at the federal level, but states and municipalities can certainly invoke such requirements.