I was viciously bullied from 3rd grade through 6th and eventually stopped it through physical violence. The guy who bullied me (we exchanged black eyes on multiple occasions) ended up being one of my best childhood friends and I attended his father's funeral last year after he was killed in a tragic accident. Fighting amongst boys is normal.
But yeah, it's true. One could argue that the school shooter was doing this for himself, which of course isn't something I'd wish on anyone. I think that these teenagers feel helpless, without any purpose in life. I'm not saying it's the worst time to be alive, but the mass appeal of no-religion, being agnostic is a real problem. I even doubt if I should continue being agnostic at times. Every once in a while I lose purpose and really haven't found a good way to cope with it.
Generally speaking, I think the cause of these shootings comes down to the feeling of helplessness and not feeling useful to society.