The whole "what could have stopped X" question is a loaded one. But regardless, the answer is gun control, and U.S. law should learn from modern German law:

https://www.standard.co.uk/news/world/germany-gun-control-laws-a4366996.html

https://mediabiasfactcheck.com/london-evening-standard/

It's crazy how even this right wing sources seems to understand that gun control is necessary and a requirement for low gun death rates, given that they admit right at the begging of the article that they have amongst the lowest death rates out there.

https://en.m.wikipedia.org/wiki/List_of_countries_by_firearm-related_death_rate

Total:

Germany: 1.04/capita

United States: 12.21/capita

Homicide only:

Germany: 0.06/capita

United States: 4.46/capita

If more guns & lax gun laws made us safe, we would should expect to see the opposite. Yet we don't, because anybody with half a brain understands that a tool whose purpose is to kill as easily as possible will make killing easier when it is around untrained people/people with insufficient reason to own it/people who store them poorly.

That's a 75x smaller gun homicide rate. We aren't going to get that small of a rate without gun control.

Inb4 somebody calls me a troll despite putting effort into this: fuck off

What “expanded” or “universal” background check applies to stolen handguns? Maybe, maybe, you could find something that would have prevented Miller, who is 18, from possessing a gun, but it was Mays, with his stolen firearm, who began shooting first. So, where are we now?

Well I'm not really a gun control guy in the normal sense but this seems like a deliberately obtuse question. By obtuse I mean stupid. If there were less guns laying around everwhichywhere then there'd be less guns for the stealing. That doesn't seem so hard to grasp, but apparently some are mystified.

It seems the argument here is that crime and violence cannot be legislated out of existence. This is like saying (and I'm not sure if this is a strawman,): All desease can't be cured, therefore doctors are useless!" What I mean to say is that it's a stupid all-or-nothing argument. There's no possibility of reducing death and violence, there's no room for compromise. But that's American politics these days, don't compromise on nothing, never.

I am probably in the minority, but I don't think the issue should be framed as either/ or.

Let's ask a similar question: what measures to oppose illegitimate voting would have stopped Trump from being elected in 2016?

Wait a second though, wouldn't denying the right to vote to a wide swatch of people be unconstitutional? Sure, if you want to prevent certain terrible people from being elected and the only tool you're trying to use is regulating voting, then you're going to have to prevent people with certain ideological views from voting. But wouldn't that go against the core principles of the US constitution...?

So what would have worked, if you don't want to touch voting rights, or eliminate civil rights regarding speech and press?

That's a bit harder, isn't it?

So, let's try this again: if you want to prevent gun violence, what would work that doesn't infringe on constitutionally guaranteed civil rights?

(And a note here: I did not and will not vote for Trump, any MAGA supporter, any christian nationalist, or any politician that supports gov't censorship of any kind for any reason. I'm deeply disappointed that, whatever other social and economic disagreements I have with traditional conservatives, we can't even agree that civil rights should be absolute.)

It doesn't matter, there should be no gun control. Government can fuck off