Earlier quoted context omitted.
Perhaps it could also be at least partly related to the systemic and indiscriminate killing of people of color by police forces across the entire country. I can't say I blame would-be "good cop" cadets deciding not to join the force to be associated with these acts; the willingness to protect bad cops by leadership seems, from the outside, to mean the entire system is rotten and impossible to change from within.
Well that's certainly the narrative that drives it. However for something to be systemic, it has to be happening regularly everywhere. The people who promote this narrative can only really point to a couple of unjustified office involved shootings (which are of course a tragedies, but are in no way 'systemic'), and in reality spend most of their time complaining about perfectly justified shootings as a scapegoat for…
The reason "the narrative" exists, I think, is that there's a widespread perception that regardless of the circumstances of the shooting -- even if the evidence clearly shows the police shot an unarmed victim in the back multiple times, for instance -- the police tend to get off on the grounds that they "felt threatened." This contributes to a feeling that the police are held to a lesser standard than non-police suspects, rather than the same standard.