So many factors at play, the elephant in the room is the Internet where sites like 4Chan flourish that provide a perverse sympathetic support and encouragement group for social outcasts that makes mass murder seem a rational and even a heroic option. You spend enough time on websites like 4Chan and even GFY at times, anti-social speech and behavior begins to seem 'normal'.
Guns are an issue but the root causes are far more complicated and nobody wants to deal with those so the easy thing is to just blame guns.
The US is a large country, if only 1/10th of 1 percent of its young population is affected by violent video games/movies, psych medications, vile social media/networking sites, etc it adds up to a pool of thousands of young male ticking time bombs.
I'm beginning to think that gun ownership should be restricted to people over 30, we don't let 13 year olds drive cars even though they are physically capable - almost all these mass murderers are teens and 20-somethings.