*sighs*
As others have said, it is uniquely horrifying to watch this while knowing what the animator would later proceed to do. However, I am not so sure that statements on 'other kids' affecting him are entirely unbiased.
Sure, it's obvious that he was a loner and was teased for being offbeat and having his own style. The fact remains, though, that it was still he that went on a shooting rampage because of it. Countless teens are subjected to horrifically cruel and ruthless teasing and bullying- abuse, really, just with another name- yet they find a way to cope, even when no one will help.
No matter how much torment someone is put through, no matter how many people afterward harp about 'warning signs'... there is still NO excuse for the random slaughter of human beings. All his actions say to me is that he could not find in himself the maturity and resolve to be the bigger man.
... and this is coming from a high school senior who has been teased cruelly her whole life... so I know how it feels.
I pray that the victim's families will know peace.