Where are the homeless kids taking revenge on the system that failed them? Even the kids that are severely bullied just kill themselves.

These shooters are always "good kids" and I have no doubt their parents tell them so, constantly. They think they're special and perfect and when the world doesn't go their way, they deserve vengeance.

Bingo. It's not the poor kids who have nothing. It's not the violent thugs. It's always the same middle class, comfortable, coddled kids with loving, well-meaning, yet utterly ineffective and useless parents who are academically capable and never hurt a fly, who stand still and take punches without retaliating because they've been told violence is bad and they should turn the other cheek.