I gather now you meant people fighting back and I was thinking people who were committing the atrocities.
I would hate things to end up like the Wild West but it could escalate to that.
Oddly enough, in the Wild West, the place where vigilantism happened most often was California.
What I suspect the modern equivalent would be is quite different, especially in those states with gun liberty. The public do not want chaos, and most police now realize that the armed public will back them up.
Even unarmed citizens are making a difference with their cell phone cameras, as their videos are persuasive evidence of criminal behavior associated with a unique individual.
So any obstacle, if there is one, are the civilian leaders, and the judges. People like de Blasio, and judges that refuse to put an individual of a given ethnic group in jail or prison, are the only stumbling blocks to law and order.
And once again, armed citizens matter, because their bullets can overrule liberal mayors and judges. And it is legal for them to do so, if they observe a felony in progress.