I believe it is harder than that
Take columbine, you have 2 deranged, ostracised, bullied and emarginised youths that have been so excluded that are completely detached from society.
At this point, once you are out, anything is game, because why would you not attack something that you are being attacked by.
It's though.
And i don't condone massacre, but i also blame a society where a bully is cool