Perhaps the shooters are not mentally unstable. Perhaps they are the sane ones. Perhaps it is the rest of us who are deluding ourselves that there is still meaning to life and moral objectivity. The western world is subject to the nihilism of existential capitalism. Self-gratification is the overwhelming ethos of the rich, famous and powerful. God is either dead or is so nicey nicey that she's not going to condemn anyone for their actions. Life is ridiculously unfair and almost all of us live perilously close to the breaking point.
What surprises me is that there are so few random mass killings. I mean, if there is no hell below and life is hell, why not?