This isn't directed at anyone... I firmly believe that if you can be trusted in society, you should be able to purchase a firearm. Too dangerous to own a firearm? Why is that person roaming free? What if they grab a hammer, or a knife, or a rock, or a tree branch?
Unfortunately the mental health system in the US is completely inadequate to handle locking up every dangerous nut, or even a decent amount of them. Prisons pick up a lot of the slack, which is pretty inappropriate. I remember reading something like 90% of prison inmates get their only mental assistance while in prison.
Stopping the idiotic war on drugs and diverting those funds to help fund mental health facilities would be a nice start.