Yeah, you should maybe read more Chinese news, and see what goes on in China. Check this out: https://duckduckgo.com/?q=chinese+mass+murder+knife&atb=v109-6__&iar=news&ia=news

First article is about "another" mass stabbing/slaying that killed 29 people and injured 140 in one go.

I don't think gang violence has really changed. Maybe it has lessened a bit. It's just a natural consequence of people who feel alienated ganging up to defend themselves and try and help each other out. There's obviously a lot of corruption and backstabbing that goes on as well, but that's the general idea as I understand it. Then you get in to turf wars and so on, and selling crack and wanting to control those areas to maximize profit. Crack which is ironically coming mostly from the CIA.

Is there a way to test this idea? If so, then where is the research, and who funded it?

Yes there is, but the research is of course suppressed by the companies making all this money, as well as those companies paying the media to look the other way. The public doesn't really hear about it, it's an "open secret". I'm sure we could dig up papers that support my assertion here, it's not exactly a controversial theory about how SSRIs work, I'd even wager it's the default stance of the psychology community. That's why not everyone should take SSRIs, but of course for profit reasons there's a lot of pressure to prescribe it to anyone who will accept it. Same with opioids.

There's so many compounding problems that would have to be systematically addressed all together to really make significant progress.

I think it all boils down to a bad economy. People do bad stuff when their back is against the wall and they're struggling to survive. I think that's the root of a lot of this. Plus our culture is being overwritten by our media, with pro-corporate anti-human values, so it's hard for the culture to even wake up to what is happening unless the media points it out for us. Which they generally won't, because they're paid not to.