Maybe a better answer is that we need to treat our crazy people. It's not a matter of expense; either you pay money to treat mental illness, or you pay money to clean up after them. I'd rather spend on getting them help than in burying our children.
Regardless of your stance on guns, this is a great idea.
I can go into a store today and buy everything needed to blow a building to bits. Remember Oklahoma City? If you don't want a big boom, you can always go the bleach and ammonia route. If you want to kill or maim people in mass quantities, you don't need a gun. You can use a car. Or a plane. I suppose banning planes is next?
So by your logic we should legalize bombs?
Of course you can't flap your arms and fly to the moon. After a while you'd run out of air to push against.