There is a better solution to weapons legislation, arming teachers with guns, "TSAing" every public venue, and futility with mental health resources trying to identify potential killers. No need to infringe on the First and Second Constitutional Amendments, either.
You can own or sell all the weapons you want. You are free to accept your children's habits as you see fit. You are free to disseminate any visuals you desire. But if a socially rejected, depressed, violent video-game watcher ambushes multiple victims, legal prosecutors would go after all persons and entities possible as secondary accessories to the murders.
Thus, the parent who doesn't have that very secure weapons cabinet, the black market assault weapon seller, the Hollywood producers and stores involved in heinous video games/movies, and the health professionals/relatives/friends that don't make really concerted effort to help that isolated, strange-acting young male could all be held accountable in a massacre. It would just be the "crying fire in the theater" philosophy regarding rights and responsibilities. There would be no big legislative actions required from politicians.
Sure, it would be hard to prove negligence and direct cause by otherwise law-abiding citizens. But a slew of publicly announced charges, costly attorney fees and maybe a few court sentences of $25,000 fines and community service labor hours could be enough to be big deterrents for everyone.