I think the root cause of any mass shooting is unrestrained anger. The anger builds up so much to the point that the person feels like it's worth his life just to let it have free reign, and above all, have the sensation of power and control.
And this is the problem--we all have anger. Every one of us. And in each of us it feels like this powerful, unique fount of power--but it's not. Your fount of power is just one of ten billion, and the only way you can just put it round the way you want is if you're a horrible dictator like a Saddam or a Kim or Stalin, etc. Most of us don't get that option, and it's really not a very good idea anyway.
Ultimately, that level of anger is always about the idea that I should not have to deal with this shit. Which of course is nonsense.
Even more than gun control, perhaps anger counseling should become easier to come by.