The Columbine shooters, Eric Harris and Dylan Klebold had not been bullied to the point of retribution. They were bullies. Dylan had been suspended for etching “F*G” into another student’s locker. Eric had a list of girls on his computer titled “Shit List” and they’d all slighted him in some minor way. He was also really into Nazi Germany and would give the nazi salute and said “Heil Hitler” a lot. They made fun of fat kids. They made fun of kids with speech impediments and disabilities.
They would go out at night on “missions” and vandalize other student’s houses for having “wronged” them in some way, but really they attacked students who they thought were inferior. It started out fairly innocuous, they’d drink and go toilet paper someone’s house- but eventually they implemented fire crackers. Soon enough they were shooting at student’s houses and breaking windows. Soon enough they were setting off pipe bombs.
The local police had been contacted about Eric Harris and Dylan Klebold two years prior to the shooting. They drafted up a search warrant but never acted on it.