My two cents.
I was in Palermo last month for the European Labor Day long weekend (May 1st)
Palermo being considered by "those in the know" as THE Italian Mafia Capital of the world.

Uh... maybe. Maybe in the old days...

Yet, in the city center, the streets were (reasonably) clean, fewer gypsies canvassing than in Rome or Milan...

Fewer Police with blaring sirens than in (peaceful) Rome.
Even traffic jams were... quieter...

You could walk the streets in the evening, even after dark, without fearing to be mugged or robbed.
Our host & hostesses swear they dine out and go to cinema and theater
and nobody in their right mind would try and rob you at gunpoint in the city center.
Where does the difference lay ?
Is it the food ?
Is it the drink ?
Is it that "discipline" is more efficiently enforced? By whom...

still, the streets in the city center are safe.
So whether it's Puertorican street gangs, or Mexican street gangs, or Nigerian street gangs...
...somebody should realize you
can teach them good manners...
...it takes a firm fist, but they will learn...
they will learn.