I feel as though large swathes of the US population, who are exposed to endless amounts of negative news and political soundbites, whilst simultaneously living in a culture which worships money above everything else is terribly damaging. Essentially, it has created a society so physically and mentally unhealthy, that these shootings are just one of many ways in which it 'comes out' so to speak.
That was pretty much the premise in Michael Moores Bowling for Columbine iirc