Well, sometimes gangs, or more specifically hate groups, work hand in hand with the police anyway. The police and the KKK have a longstanding tradition of having each others' backs, not accounting for potential members of both groups.
It's bad, and it's been bad for a long time. Our culture celebrates violence and guns, and I think that also influences police behavior. They start wanting to be heroes, and don't want to bother deescalating a situation.
And yea, a lot of places make it illegal for you to record a cop without his/her permission. It can be the most damning evidence out there, and you are probably going to go to jail for releasing it.
There's just no one to stop them. No one to step in and say "this needs to stop." The conservatives love this stuff because it is happening to minorities and not the wealthy white upper class.
The supreme court has even ruled that the police are under no legal obligation to protect you. I read an account of a guy who was stabbed by a serial murderer. The cops hid in the trains front while the guy went all stabby on him, and he ended up taking the murderer out on his own, followed by the cops sweeping in to claim victory. He sued the NYPD, but because of that ruling, he had no case. The cops can sit back and watch you getting raped or dismembered or really anything, and if they don't want to step in, nothing requires them to do so.
Because of this, the officers that watched another officer rape a little girl five different times in abandoned buildings will face no punishment at all.