Quick question. How many black people did US police murder last year? Order of magnitude. How many would be few enough for you to say that the police have "stopped" - 100? 10? 1? 0?
My guess is that the actual number is a lot lower than you're thinking, even when counting unintentional things like "police officer made a mistake, shot innocent person" and "pig strangled someone a little too long". Not to say that that's excusable when it does occur, but...
The thing that goes after the but is called an "excuse".