I actually think your judgments of the slogan are entirely backwards. ACAB is precisely about understanding that the entire class of police officers are harmful to society. Human beings are human beings. When they put on the uniform and act in the role, the act in the role of cops, and become the C that are all B's.
At the very least, ACAB got a lot more people understanding that police forces themselves are the problem, not "a few bad apples."