Fear - the most concise explanation I've read so far for all these shootings. Paranoia, fear, mistrust of the government, of one's own neighbors, of one's own family. You nailed this one on the head.
That fear is bred by our divisive politics and our sensationalist media. I blame the news more than I blame guns and gun ownership. I think gun ownership is another symptom of the problem - not the problem itself. Banning assault weapons won't work.
What's the solution? Funding public mental health hospitals and asylums are a step in the right direction. Treatment over incarceration is the right direction as well. Psychological testing of school children might help identify potential problems and get kids the help they need before it's too late. Finally, ratchet down the political rhetoric.
Ultimately, though, history shows that whenever we have a dramatic shift of how society functions, we have an increase in violence. Either we have a war (American Revolution, American Civil War, etc) or we have riots like in the 60's, anarchist bombings like the Haymarket Riot, assassinations like Lincoln, or many other examples throughout history. Unfortunately, we live in turbulent times currently, where we have a national conversation on moderately changing the entire direction of the country - changing how we operate for the first time since Jimmy Carter left office. Naturally, it's going to lead to a lot of fear, and as a result, a lot of violence.