They banned lawn darts when I was a kid based on the potential for harm.
The only reason he’s not committing a crime before he starts shooting people is because the gun is legal. You are bootstrapping. You could just as easily outlaw the gun and it would be contraband, and then he would be committing a crime prior to going on a mass murder spree.
What I’m suggesting is that humans are simply too frail and prone to decompensation and loss of control to let people have whatever weapons of war they want.
If I had a magic wand I would come up with a formula for lethality that measures stopping power over sustained fire for maybe a minute or two, taking into account the time it takes to reload or change magazines.
I’m all for self-defense. I could even buy into an interpretation of the Constitution and of natural law for that matter. That gives everyone a right to possess self-defense weapons. If you can’t do the job of self-defense with five or six shots, you got a problem that no gun will solve. Why should society bear the burden and risk of giving you those guns anyway? The risks outweigh the benefits.