The argument for ACAB is essentially that as a condition of employment, cops are required to enforce unjust laws, making it an inherently immoral profession even for those with the best of intentions.
They're also required to enforce laws that are just and should be enforced... by police. Realistically speaking, the profession is not just inevitable in modern societies, but necessary despite it's flaws and the need for reform of it's current structure.
There is a moral component to the job that requires one to do both just and unjust things from time to time that's baked in. Arresting a murderer or rapist or thief? Good things. Arresting a non-violent drug offender? Bad thing.
Police brutality is far too prevalent, but it's not a necessary part of the job engaged in by 100% of cops. Unfortunately, the culture of the blue often leads to otherwise good cops protecting the bad, which is itself a bad cop (in)action and I don't know of a good way to change or even quantify that.
By all means, we should end qualified immunity and the drug war and no-knock raids and police unions and the overall militarization of police. But I'm not ready to blame every cop for every wrongful shooting much less disband the police.
Most professions are morally neutral in that they lack a moral requirement one way or another. When it comes to policing? It requires both moral and immoral actions as part of the job... but it does require both.