It appears we all agree that mentally ill people should never be able to purchase a gun ever.
They should be institutionalized. A gun is one of about a thousand things they shouldn't be able to buy.
The thing you have to remember about mentally ill people, FSD, is that they typically aren't diagnosed until
after they do something really crazy/violent. Easy access to semi-automatic assault rifles (not handguns, even though most people in america are killed with handguns, separate issue) allows for a mentally ill person to announce their condition to the world in a really brutal way.
If you refuse to budge an inch on decreasing/eliminating the availability of assault weapons, then you better have a super fantastic minority-report style way to screen all the crazies out before they do something terrible. I'd like to hear it. Please enlighten me.