What percentage of homicides and violent crime are related to gang activity? Take Chicago for example as it continuously pops up in the news. I would contend that better mental health care does nothing for them.
I take a broader view of mental health besides just treating pathology. I see it in more holistic terms, that involves poverty, debt, external stress, family life, even nutrition and physical health. People without purpose in life can find purpose.
There's a reason you don't find violent street gangs in Lake Forest or Kenilworth which are as close to downtown Chicago as the far South side. And before the ACs jump in, no, it has nothing to do with the properties of race.