Newsflash: a lot of kids are picked on in school. I can remember many times when it happened to me. I can remember it happening to other kids. This story suggests that Cho was not responsible for his actions, but a victim of his cruel classmates.
Make no mistake, I think it is cruel for children to pick on each other, but that is children being children. You learn to get through it, and learn that those who tease you tend to be pretty insecure themselves.
Understanding this situation is one thing; but to talk to old classmates and look for evidence of teasing is irresponsible. Cho made a decision to isolate himself (there is always someone willing to be friends around), and he made the decision to go on his rampage.
There are thousands of real victims out there; why not focus on them for now?