Debates about violent video games and right to carry arms will rage on forever and nothing may ever get done other than a lot of quibbling. The bottom line is, are you willing to give those things up, lets say for 5 years, on the chance that this sort of thing will stop happening? I am.
Ban all violent video games. Remove the right to carry a gun, or even own a gun. Wait 5 years. Count the number of mass murdering that goes on. Then, go back if it didn't work. If it did, we all win.