I think bullying breeds school-shooters
The vast majority of school shooters in America were badly bullied before they decided to shoot-up the school. That can't be a coincidence, can it? Well, to be fair, most bully victims will never do any harm. However, if a bully victim is mentally ill, bullying can be a trigger that unleashes a school shooter. So that combination, mental issues + being a victim of bullying, that is what breeds school-shooters.