[hurrdurr56]

Well, it ultimately depends on how you want to define insanity.

Most of the school shooters aren't schizophrenic. They usually aren't complaining about hearing voices or hallucinating. When the media claims they had a "history of mental illness" it usually means mild to moderate depression, anxiety, ADHD, things of that nature.

Some of them will claim to have a political or ideological cause but most of the time their manifesto (if they bother to write one) consists of something like "Everything is meaningless bullshit and I am going to prove it by killing a bunch of random people for no reason".