Poorly chosen slogans seem like they really hurt the movement. But maybe the ACAB crowd truly does think all cops are bastards.
I once had an interesting discussion with someone about this who had obviously thought about it a fair bit. Their position was that the institution is set up in such a way that your own intentions, regardless of how good, are systemically constrained into a narrow channel and the function of the job is to be "a bastard".
In other words, it wasn't a commentary of the nature of the people in the job, rather the nature of the job.
It was equally clear that he expected that from them, and that this was not his first case of abuse. This was merely the time when it became incontrovertible, because somebody had a camera and was willing to take the risk of filming it.
If they aren't going to stand up against abuse even when it's literally a murder caught on video, there's no way to avoid the conclusion that there are indeed countless other abuse cases that have been covered up, with nobody willing to stand up to it. It doesn't show the extent of the abuse but it shows the extent of police willing to cover up and excuse that abuse: universal. That makes All of them Bastards.
That's like thinking someone might believe “one bad apple spoils the whole bunch” applies to police, or that the “broken windows” model of policing has the same cultural effect directed at the police that iats advocates say it has directed by the police at communities.
Or, more generally, that people’s ethics and conduct are responsive to the institutional incentives and cultural environment surrounding them.
Clearly, this is implausible; no one could believe any of that.
That said... the cops who made the news for shoving other cops off from kneeling on protesters' necks? They were good cops in that moment. Even if they were doing it to protect their precinct from scandal.