"The basic fact is that the guy was nuts and there is no logical explanations for what a nut job can/will do. We try to superimpose logic and reasonable thinking onto an illogical unreasonable person. It doesn't work."
It is hard to deny, though, that the shooter had to have harbored a lot of anger at his parents. Whether it was justified anger is a different question; regardless, nothing can excuse the way he expressed that anger.