many of them seem to be ex-military, so they would have been people who at one point did have a chance to have an external enemy (no I didn't say the converse and that a great deal of ex-military are going on shooting rampages or anything like that - suicide seems to be the main risk there). So maybe these few deeply disturbed people can't get that in their lives anymore? Wow sad.But maybe this internal strife, the ongoing shootings, is our violence/cross to bear. Maybe humans have internal need to have violence in their lives. There is a theory of war that war allows acceptable outlet for killing, raping, mayhem for those who have this need. That is not at all people of course, not by a long shot, and our military folks are largely not of this mental mindset.
I don't like the generalizations about maybe humans have a need for violence etc. as statistically the overwhelming amount of such violence is committed by males, so more a case of who you calling "we"?