I’ve written in Pharyngula comments before about something called the Pseudocommando shooter type. These are the folks who get big rifles with rapid fire capability and carry multiple weapons and huge number of rounds of ammunition to a target location and then shoot as many people as they can. The psychological research on the type shows that serious mental illness is uncommon in such shooters, though they do show an above average rate of mild to moderate depression and a couple of other of the most common mental illnesses. Some of the research that initially characterized the type found even these mental illnesses affected only about 40% of such shooters.