Good question. As far as i'm concerned, the defense of the slogan ACAB relies on a deeper and systemic reading. a) all cops are bastards because being a cop necessitates following rules and procedures that have nothing to do with helping people, even if that's their justification. that doesn't mean that every second of every day, cops only do bad things. b) cops are not just the people paid by the state to be cops, they're also the people who follow rules and procedures of other dogmatic sorts (not just the state's), so anyone can be that kind of a cop, including anarchists, and so we want to "kill the cop in our head" (a May, 1968 slogan from paris).
i know some cops who've been very nice in the moment to me. But in general they are forced by the system and by each other (among other things) to be horrible to people who are not cops.
there's also the reality that having specialists for dealing with the most violent and deceitful groups of people is inherently destructive to that band of specialists. i did a ride along decades ago with a cop who could only see me as a victim, because he saw me as a nice white person, and he only saw those in his job when they had been victimized. so that will make people be bastards too...
on the other hand, as with all slogans, there will always be people who take them at their most simplistic and moralist (cops bad/non-cops good style).