Then you kick him out of the house.
My son is 20 and perfectly sane. But if he bought a gun, he'd be living elsewhere. My house; my rules. That's what good parents say.
Maybe there's some sob story about how they tried to get him help, blah, blah, blah, but I highly doubt it.
Boundaries are good when dealing with people living at your home. Even more important when that person is mentally ill.
I didn't think I could ever call the police on my kid, but I did.
Point is - it is amazing what you will and won't do when your kid really needs you.
I am not going to judge his parents. It's too easy and does me no good. The incident that happened in Tucson is one of my greatest fears. I know no matter how great of a parent I am, things may be out of my control.
I am sure his parents are in horrible pain right now. It's a sad situation all the way around. Sometimes there just is no silver lining.