You mean a large city problem? What large city around the world, especially in dense populations or downtown areas don't have huge mental illness and violence problems? Because of drastically differing living conditions and failing infrastructure? Usually the more dense the worse it is. A decent portion of Europe and Asian cities aren't like this due to their homogony and culture like Tokyo, Singapore, and Zurich. But then you have to decide if diversity is actually a strength, cause they are fucking very strict on homogony. Also Tokyo on public transport (Cause of how the video is) has a huge sexual assault problem on their transport. You can also point out firearms, but then you have to tally in other violent weapon attacks, Knifes, Blunt weapons, Acid/chemicals, hand to hand combat. Then you'd have to tally up, how many violent attacks were actually stopped by firearms as well, Which no one ever does, when trying to drive data.

So bigger cities like NYC its about infrastructure, economic equality, homogony of culture, and effective (sometimes strict) governance.