Ok well I am unsure of the statistics for mass murderers but I do know that over 70% of serial killers are abused as a child. I can't imagine the stats being much different for mass murderers. While that is not justification it does go a long way in explaining the thought process behind people like this. The reasoning can be anything from being bullied, abused, made fun of, left out, to insane. However if anyone decides to commit a serious crime over these issues I would say that had to have some sort of mental illnes. The normal thought process is to feel anger and remind ourselves that revenge is not acceptable. i.e the thought process of a mass murderer is not normal.
They live in a state that has a tradition of violence being used to solve problems. It goes back to the 1870s and the days of the wild west. I'm sure he had some reason for doing this other than the simple answer of being crazy. Maybe he hates people for some reason? We'll never know unless we ask him. But I'll bet no one does that.