At the most fundamental level: personal responsibility. Some folks are just irresponsible, while others have mental health issues that render them violent or otherwise unsafe to own a weapon of any kind. There are always a small minority in any community to ruin things for the rest of us with their poor behavior. Guns are no different.
Exacerbating the issue is that America is supposed to be a "free society" where we cherish the concepts of due process, innocence until proven guilty and guaranteed constitutional rights for all. Not to mention certain inalienable human rights.
So, if we can't suspend a persons constitutional right to own a gun until they commit a crime or otherwise have their gun rights revoked by a judge via due process under the law, the how do you stop someone with no criminal record from shooting up a school?
And, if we attempt to ban the tools, it simply won't work, IMHO. Government has never, ever been able to prohibit any object or substance or activity if the demand exists. But we could try real hard, convince law-abiding gun owners to give up their guns, outlaw more and more types of guns, and those who are willing to ignore the laws will still be armed.
Like I keep trying to remind folks, there are already tons of laws against murder, but that doesn't seem to stop folks who are intent on killing. How would any degree of a gun ban be any different? Especially considering how many guns are already in the hands of civilians.
Many developed societies consider themselves to be "free societies" (and in every case that is arguable, including the US) where the things you talk about are also cherished. Can you please stop talking as if the US's principles make it unique? You're pretty much the youngest developed nation in the world and sometimes the way you talk sounds a teenager who's telling their parents that they don't know what growing up is like. The fact that you have a Constitution does not make you unique, your history has already demonstrated that the Constitution can be changed which makes it just like anything else made by man.
I agree with you entirely on what the ('fundamental') problem is, but you haven't answered my question of what the ideal solution would fix. Also, you keep talking about bans, but frankly using the word 'ban' carries a lot of absolutist baggage that most countries have not adopted, including the UK.
Furthermore, the way you describe the 'fundamental' problem to be (the kinds of problematic people that have easy access to guns), you even go on to portray the Constitution as being a barrier to solutions. So do something about it? In your response to Pipeline 1010 you even cite the constitution as the only reason why you don't like his idea.
IMO your problem is that you describe a fundamental issue and yet can't recognise the fundamental cause of it: the default being that everyone then has easy access to obtaining firearms. The solutions you proposed earlier often revolved around the idea that you can catch problematic situations (such as worsening mental health) just in time (that's an argument I could drive a steamroller through), and even worse from your perspective I think: On one hand you talk gun ownership being a Constitutional Right (implication: set in stone, no-one can take my guns away, yadda yadda yadda), and in your earlier solutions, it very much sounds like you're treating it as a privilege that can be taken away.
If you want to continue to use the Constitutional Right argument, then I think you ought to accept that fundamentally you have to treat 2A like 1A which in America AFAIK has exceptions that are so few and minute in scope. I don't think it's a realistic view to put 1A and 2A on equal footing when you then talk about classes for gun owners. Can you imagine having mandatory classes to facilitate proper use of freedom of speech?