From playing cowboys and Indians in the 1950's to playing videogames to kill hundreds of aliens or humans, violence has always been part of play. As in dealing with fairy tales which have bad wolves and witches who bake kids, these rituals desensitize children to normal fears of life. News always included war, at least somewhere, and genoicide happened somewhere in every generation. Of course, did the media spell out every detail of every going-postal-type incident over and over again? Some "crazy guy" shot up people in a subway or a building or terrorized with a bomb somewhere, ever here, every now and then.
When we have several mass killings in a row, we start analyzing our mental health and gun policies. They're never adequate enough, for sure. Neither is family life securely guaranteed and childrearing falls short fall too often. But, most people with mental health problems and most gun owners, most isolated and abused children, most spoiled children, aren't violent.
So what is the cause of all the shootings by anger, disturbed people this decade? My guess is contagion. Like suicide becomes popular for while in a high school once two people do it, like sex abuse multiplies when someone gets away with it in sports or church, means of creating tragedy catch on when they are popularized in the fiction and real media. It's sickening and it would be great if we popularize helping people, being sensitive, making peace, creating art, talking about relationships. We do try but let's try harder.