I want to call out something, since I feel a connection is being drawn here that is not valid. His gun was not 3D printed.
Investigative sources said that the gun used in the incident consisted of two metal cylinders wrapped in vinyl tape, which could fire six projectiles when the trigger was pulled. When prefectural police examined the seized weapon, they found it was equipped with an electrical cord and battery and that it was designed to ignite the gunpowder with an electrical current.
Sourced from this site: https://mainichi.jp/english/articles/20220725/p2a/00m/0na/021000c
I'm not taking issue with most of what you said, but FDM 3D printers aren't the issue here. Making a gun can be done at home with essentially no tools. I don't think we should get pulled in to blaming societal issues on a single piece of technology.
I'm in a hurry so I've left this comment short, I may come back and edit it with more thoughts later.
So, like, I agree with you. We need to make people less miserable. We need to find ways to make people feel less marginalized through better representation, and a reduction in authoritarian measures. I also feel that ending the cycle of abuse that leads to damaged children growing into harmful adults should be prioritized and addressed with better social services (free healthcare including mental healthcare) and societal reforms. You and I fundamentally agree on this, and everything I'm about to say below is the result of me being in a bad mood because my FIL shared some fucking stupid racist GOP misinformation with me about the cats and the Haitian people. I'm being pedantic (and frankly irritating) about this, but I'm just not regulating myself very well today.
I guess I'd question how much more accessible 3D printing is making it. Like, there are definitely gun parts you can 3D print, but they tend not to be the important bits. You can somewhat successfully 3D print the important bits, but that requires a lot of knowledge and skill (printing with high strength/high temperature plastics like Nylon or polycarbonate isn't easy, even with the newest crop of printers), plus a good printer (which is either expensive, or was self-built which increases the knowledge and skill required). If someone wants to make a gun with a 3D printer, they'll have to get a printer (and either spend $1000+ or spend $500 + months making one), learn how to use it, learn how to print with better filaments, buy all the vitamins (i.e. parts that can't be printed) and then they'll end up with a kinda shitty gun. Alternatively, they can do what Yamagami did and buy some cheap metal pipe and a battery and some tape. They'll get a kinda shitty gun without all of the 3D printing hassle for much cheaper.
Hell, if someone wants to make a good gun then machine tools have never been cheaper. I have a metal lathe and milling machine sitting out in my shop right now. I do not (and will never) make guns, but I could. My dad does competitive target shooting (Palma rifle) and has asked me if I could machine stuff for him which is why I've thought of this (I told him no, btw). My lathe was built in the 1950s and was $500. My mill is more expensive (about $4000 brand new), but I'm sure you could make a gun with a used $1000 mill. Machining is harder to learn than 3D printing, but with machine tools you can make more capable guns (i.e. semiautomatic/automatic, accurate, precise, reliable). The drawings are out there, and I'm sure I could turn some 4140 and O1 steel scrap pieces I have into a little gun. I can even make my own springs with my lathe. I have a granite lapping plate so I can make parts that are incredibly flat and smooth. I have a MAPP gas torch that I've used to harden and temper tool steel. A small home machine shop can produce a gun that's as good as anything from, say, the 1940s.
Should we say that companies like Harbor Freight or Grizzly are making homemade guns more accessible because they're selling cheap lathes and mills? I don't feel that's fair. Our technology and automation is getting better in general, which is making guns more accessible. It's why Yamagami was able to kill a world leader with parts from the hardware store. We can't really do much to stop the hardware store problem. All we can do is what you said. We can remove the things that make people feel the need to own a gun.