When people group together (form a society) they tend to develop something called "rules of civilization". These rules are all the little accepted social quirks that identify someone as A) Part of this social group and B) behaving within acceptable parameters.
Put simply, it was a group-survival defense mechanism.
Along came "political-correctness" and our current litigation-happy society. Now pointing out that someone is behaving on, or well past, the fringe of acceptable behavior will get you a lawsuit and other problems.
So people are getting re-programmed to just "mind their own business" and the natural protective and corrective actions of "society" are being impeded.
So is it any wonder these fringe cases are not being handled or even acknowledged until it is too late?
Combine that behavior with the fact that studies have shown that a certain tiny, but astonishingly consistent, percentage of ANY population is "fringe" and multiply that percentage by a population of 300 MILLION people.
Now add in a dose of economic hardship and the stresses involved.
I'd say that's a pretty explosive mix of elements in it's own right irrespective of guns, bombs, knives, or automobiles.