He can kill more people with his car than he can with guns.
Exactly. As much as I don't think the severely mentally ill should have firearms, if they are free citizens you can't disarm them for that. If they aren't a menace to society without a gun, adding a gun into the mix should not change matters. The mentally ill don't need guns to kill people.
And the fact is, most of our mentally ill are in prison already. Sure there are many living in the world among us, but the same can be said for criminals on a much larger scale.