I think the issue we're discussing here isn't "sanity," but "mental stability and ability" of a sufficient degree to where the individual poses no unlawful threat to themselves or society at large. I pose a HUGE threat to criminals, but only if those who choose to put themselves into the few legal situations, threatening life, limb, and property, where I would then have the right to use deadly force to stop the threat. Whatever threat I pose, it's lawful.
The problem with mental stability and ability, whether it's a hard-wired issue within the brain (schitzophrenia), induced by situational/social factors (PTSD), chemicals (alcohol, drugs, medication), fatigue/alertness, and just plain old education/common sense, is that it is not a black and white issue. It's a continuum, and most of us are adversely affected by at least one of the above subsets of factors, if not two. The question is, "to what extent?" Am I affected to the extent that it will adversely affect my own safety or safety to that of others?
Living alone in an apartment and in a State with a solid Castle Law, my only real concern is whether or not I'm mentally/physically capable of safely handling a firearm. At least at night, where an accidental discharge could injure or kill a neighbor. Still, during that time, if someone enters my apartment while I'm sleeping, they're in serious risk of catching lead. During the daytime, I have to consider the likelihood one of the maintenance workers might knock on my door and, if I'm in the BA/shower, may enter without my knowing about it! I'm obviously not going start firing without seeing who it is. Furthermore, while my son is visiting, I know he sometimes gets up in the middle of the night, so I want to make sure he's safe, too.
I'd rather take a bullet than put one into someone who is innocent. Thus, if I have any doubts about my own ability to safely be in possession of a firearm, my firearm stays in the car, stays at home, or even gets locked in the safe.
Thing of it is, I'm not a mental health professional, and while I know there are many factors that can result in a person firing on someone who is innocent, I'm not qualified to judge when that might be. The problem is, I'm not so sure they are, either, except in clear-cut cases. If one has to err, on what side should they err? On the side of the gun-owner's right to keep and bear arms? Or on the side of the general citizen's right to be secure in their persons? Thing of it is, "the right to be secure in their persons" is a misappropriation from the Fourth Amendment, which states: "The right of the people to be secure in their persons, houses, papers, and effects,
against unreasonable searches and seizures..." Meanwhile, "security of the person is a basic entitlement guaranteed by the Universal Declaration of Human Rights, adopted by the United Nations in 1948." -
Source. It is NOT a Constitutional right. Where it is found is in the Declaration of Independence's "right to life" clause. That's about it, although countless laws on the books about the various forms of murder, manslaughter, and mayhem make it clear that harming another human being without lawful authority is bad ju-ju.
My gut feeling is that the RKBA should be afforded to all adults and denied only to those adults who by virtue of their felonious or mentally aberrant choices of behavior, have put themselves into a category that poses "a substantial and unlawful threat to themselves or society at large."
Now the question becomes, "What's substantial?" Felony criminals comprise less than 5% of the population yet are more than 20 times (that's 2,000%) more likely to cause harm to your average citizen. So what should be the standard? The same increase in risk as for your average criminal? Half that? Twice that? Sadly, I don't think we can accurately predict it.
On the other hand, I don't think we should even try. Life itself is not without risk. I have no problem telling someone who committed armed robbery, "That's it - you forfeited your RKBA when you made the bad choice to rob the liquor store at gunpoint." Perhaps if there were extenuating circumstances (being forced/coerced to hold the gun by gang members) and a lengthy record clean of criminal activity while full of progress (degree, success in business, etc.), I'd be willing to reinstate the fellow's 2A rights.
Meanwhile, I have very serious reservations about telling someone, "Based on your pattern of lawful but risky behavior in college, we feel you're at elevated risk of mental stability and therefore pose an increased risk of being a threat to life and limb in the future." There will always be people among us who are mentally unstable. That's just life. If their observed behavior i.e. injuring or killing people says they're a threat to society, then they should be treated as a potential threat. But to say someone might be a threat and reduce their rights based on a probability, my own thought on this approach is that it is itself a bit insane.