The Columbine shootings didn't happen as a result of bullying. The shooters themselves were bullies in their school
By most accounts, Eric Harris and Dylan Klebold were not the victims of bullying and weren’t socially ostracized. They weren’t the most popular kids but they had friends at school, and they actually wrote in their diaries about how they bullied other kids in school. In addition, we were lead to believe a lot of other misconceptions about the Columbine shootings: the shooters were not “goths” and weren’t motivated to kill by music or video games. They also didn’t single out Christians, jocks, or African-Americans in their school. They intended to blow up the school with bombs and kill as many people as they could, indiscriminately. Thankfully, the bombs didn't work, so a lot of lives were spared.
For more on the true story behind Columbine, try this USA Today article.