I think it's bullying, frankly, and social rejection. The cause, IMO, is what Kris says--a disconnect from peer groups aka society.
The Lacote? (Native North American kid), Columbine, the WV kid--they all were said to be either "loners"...goths....quiet kids... People don't do that as a choice. They either have an internal schism that makes people not want to hang around them--or their just don't measure up to our lovely society's sterling standards. (Ask Merry about sarcasm)
Parents can many times see this and intervene before things get awful. The Columbine parents were practically assessories, IMO. If they'd ever stuck their heads in Eric or Dylan's bedrooms, they'd've seen the hardware all over the place.
I do feel horrible for the kids, though. All of them.