U suspect the reason we have so many of these is shooter video games. The real problem isn't the pistol grip, etc. It's people fantasizing about them. Imagining themselves as a heroic (but tragic) figure, taking down hundreds of enemies on a suicide mission. They know they are going to die, in all likelihood. Maybe we need to show on national TV and the internet life in prison that school shooters serve, including all the times they are raped, stuck with shivs, etc. and guards just laugh. So these punks know what they are really signing up for.
If you want to go out in a blaze of glory, attack a police station with your AR-15 instead. Call ahead so they can have enough people there.