The problem is that the "good" police are incentivised to protect the "bad" police, and do so. You're not going to find the "good" police investigating and charging their colleagues - when it does happen, they're inevitably harassed and removed from the force - and nobody else is in a position to do so, so what you get is the "bad" police operating with impunity.

And there's effects on the wider system - courts will believe a police officer's account of what happened pretty much no matter the opposing evidence. There's no accountability when a police officer goes against the reasons they were hired, and destroys people's lives.

There's the possibility of alternative systems of protection and justice, which don't create organisations which are incentivised to protect murderers, abusers, and rapists.