Nobody is ignoring that either. Indeed, the knock-on effect of BLM being successful would be that the police in America killed fewer people for no reason full stop.
Moreover, the issue of general police brutality is shocking, but when it's brought up in this context it seems to be only to distract from the separate but entirely related issue of police racism, rather than out of any meaningful effort to solve the problem. "It's just a few bad apples", the typical defence, is followed by no proposals at all to deal with those bad apples, or to prevent the system from tolerating them in future.
I'm absolutely fine with discussing the wider issues of unchecked policing power, brutality, and the apparent general disregard for the lives of the people they are supposed to protect. I am not fine with using that to distract from an equally important issue.