I think the problem with gun control (because in this case gun control wouldn't have helped since there are no mental records afaik), is that it's a very specific city-centric debate. I don't think taking guns is a good idea when you have one sheriff department for a huge surface and you might actually need to defend yourself to stop dying before they reach the scene. In the city-centric areas, there are more than enough cops, so that's why they don't get the need for guns.
So imo it's a case of what you allow in the cities. It seems to be where they manage to make the mass murders because civillians don't actually carry firearms.
It's a much simpler explanation that also explains your noticing of a city-related pattern.
Most people don't buy guns with the intention of shooting other people, because in general most people don't have the intention to murder other people. Even in situations like Chicago, etc. where it's borderline illegal to own a gun, people participating in illegal activities generally carry because they're engaged in illegal activities and it's for self-defense as normal methods are taken off the table for them. Criminal organizations have long ago recognized that large scale violence tends to bring unwanted attention to where gang wars and gang violence dropped with crime rates, in some areas, even faster. Even
The Wire wrapped an entire season and a half of plotlines around this cost-benefit analysis within a criminal organization, but it was taken like most of the show from the real world.
So even in those instances where we have people already committing illegal acts on the regular, and generally acquiring weapons illegally, there's social pressures that discourage them within their world beyond their own desire to kill or not kill fellow humans.
When someone chooses to murder especially on a mass scale, they have an extreme advantage over everyone else who operates under the standard presumption that others do not desire to murder them. Thus the general patterns of planning, stockpiling of weapons, targeting certain dates, even doing test runs, etc. A person who has thrown out the intention to not mass murder has thrown out the playbook and circumvented nearly every tripwire so they can walk into a movie theater, school, sit in a hotel, park a truck they've loaded up with a homemade bomb, etc. knowing what they intend to do which no one else does and knowing what they have access to which no one else does and knowing that they aren't participating in any sort of traditional norms. And this applies even if they're just going to kill their partner with a nail gun to the back of the head for referring to "Disney's Star Wars" or whatever.
Consider the societal mass murder of our age, 9/11, and why it worked. Prior conditioning for
decades suggested that hijackings were generally used for hostage taking and the hijackers were not suicidal (and often not even overly violent) to that scale. The 9/11 hijackers had every advantage because they knew this was not the case and nobody else did. (Our air defense systems which did have planning for this did not have actual operations for anything like the immediacy needed as they couldn't know the intentions.)
Except Flight 93. Which changed the situational calculus of at least enough passengers on board to foil the hijackers plan. Arguably not because the passengers were any better armed or prepared or smarter but because it evened the playfield in terms of the knowledge of the no-win scenario they had been placed in. (Even if we go down the Alex Jones path of it being shot down or blah blah blah, it was the only of the planes that could be, because only by that time did the defense systems in place know the intentions and have the capability to react to those.) It was the hijackers who then became at a situational disadvantage.
