There's plenty of evidence that show a good chunk of school shooters are or were bullied :
> 76% posted disturbing content of guns and threatening messages.
This is them provably being bullies as judged by their own social media accounts.
> 60% reported being bullied in-person or online
This is them claiming to be bullied at some point in their life. Elliot Roger claimed to be rejected by women while what happened was that he self isolated entirely for whole his life. This was similar to other guys - a person with track record of violence or threats feeling bullied by other people basically existing.
Compare the two claims, really.
edit: The Secret, not The Present
He socialized basically exclusively online before going to the college.