If a couple of these start beating up a guy on the street, and he happens to be carrying and mows them down, whoooo boy. It's going to be like it was after Bernie Goetz did what he did in NYC. People are going to treat him as a hero, and they should. He'll get credit for stopping the flash mobs. Or maybe it will take two or three of these events happening until they stop.
Either way, the flash mobs are not afraid of the police. The police are enabling them.
To be fair to them, they used to. Then they were hung out to dry by the forces of political correctness. Real cases of police abuse were used as a bloody shirt to hamstring police officers who were just doing their job - "take out the garbage", as you put it.
It's tragic, but a lot of the "brotherhood" code of silence in police forces is in place to prevent them from being jumped on by guilty opportunists with a convenient bloody shirt. Unfortunately, the code of silence does cover up real cases of police abuse - but it does draw a line that fends off "Mr. Mistrial" from walking all over everybody.