It's because of psychiatric medications. Withdrawing off psych rx's "cold turkey" brings about a condition called akathisia. It's an unbearable combination of anger, anxiety and aggression. One of the Columbine shooters was on Luvox. Another very well-known serial killer (not a school shooter, though) was on Halcion.
Here's a link to an article about psych Rx's and akathisia:
This is a quote from the article at the above URL: "~ The Food and Drug Administration in 2004 required manufacturers to update the drugs’ labels to warn of the possibility of “anxiety, agitation … hostility (aggressiveness), impulsivity, akathisia” and other problems, including suicidal behavior. ~ "
The young man at the University of Virginia had also been on psych meds. Unfortunately, the press always reports such occurrences as, "The shooter went off their Rx's and so the underlying 'illness' resurfaced." That's not true. The shootings are caused by the withdrawal psychosis --- or, at times, just by the drugs themselves, even if there's no withdrawal.