If observe those school shooters , 75 percent are being bullied at school or family. Okay maybe some of them aren't like erric and harris but others yes like adam lanza and sueng hui. I think we must concern on these factors.If these are solve then school shooting wouldn't happen again.
Mass shootings are committed by people with personality disorders (i.E. Anti social, psychopathy). They lack empathy, lack understanding of social norms, often overstep others boundaries, are aggressive and deceptive. These traits would naturally be abrasive to peers who might avoid or bully this person which is merely a coincidence relating to the shooting it's self.
Mass shootings are usually cold and calculated. The person does not suddenly snap. They prepare plans, guns, ammo, bombs and so on.