I should point out that Harris and Klebold weren't bullied as much as people tend to think they were; in fact, they were often the bullies themselves. The problem was that both of them were violent lunatics who literally viewed themselves as "the gods" according to one of Harris's journals who were also good at lying to people to look like they WEREN'T violent lunatics. And when they made a video for a school project that involved them shooting students in their school and swearing and making violent threats to the camera, they were COMMENDED for it by the teacher.
That's not to say that some bullied kids don't respond to the stress through murder. Just that the most famous pair of school shooters weren't victimized outcasts, but rather a pair of Grade A nutjobs that everyone ignored until they showed up with guns.