If you know an unstable gun fetishist -- family, friend, or acquaintance -- disarm them.
If the law doesn't agree, get creative.
The life you save may be your own.
It's become socially acceptable to take the car keys away from someone who is drunk.
The same ought to be true of someone who can't be trusted with firearms.
= new reply since forum marked as read
Most especially random "intruders."
I've lived a long life having unexpected encounters with strangers in my house, most of them turning out to be friends, or family, or friends of family, and the remaining strangers mostly harmless.
dude, that's my beer!
I suspect Spot eats the truly bad intruders, but that's not my problem. Dragons will do what dragons do.
He was bipolar, off his meds and having paranoid delusions. Removed all his weapons while he was in the hospital. Some were given to his sister because they had been passed down from my grandfather. He asked for them back when he got out and she gave them to him. 
But a couple of months later he was standing in the middle of the street with a weapon, saying he was "protecting the neighborhood". The police picked him up, took him to the hospital, took away his weapons and didn't give them back. It's a wonder they didn't just shoot him, but he's white.