During my working career I worked several years for a company that made the hardware used to generate radar and land mass simulation for military aircraft flight simulators. Our product was used to train pilots and to acquaint them with given targets in preparation for a future sortie.
Before the actual attack on a given target, the pilot became totally familiar with the target, the terrain and landmarks he would see from take-off to landing.
As I see it, video games give the same type of simulation to attacks within buildings and outside, performing massive carnage. An individual who is a few French fries short of a Happy Meal can be adversely affected by such simulation and act out what he has been practicing in the video game. To him, it's just another game.
In my opinion, movies also play a role in mass murder. The purpose of a movie is to make viewers happy, sad, aroused, feel tough, be scared or feel super-human. Those feelings leave a normal person soon after his viewing. The language and graphic violence that come along with most movies, I feel, numbs the minds to the real damage and heartbreak that these acts generate in real life and thus can encourage that one person in the audience to act out what he has seen.
Mental illness needs to be aggressively addressed.
Outlawing certain types of firearms will do just what it has in the past: nothing.