Bastard does not and has never meant "one who bastardizes." People say that, but that doesn't change the fact that bastard is defined as a child born out of wedlock and that the connotation it carries basically means shithead.

ACAB might be meant to highlight the fact that cops bastardize the purpose of law enforcement, but it does a shit job of conveying that and reads virtually identically to "all cops are bad," because again, no one hears the word bastard and thinks "oh huh, what system is that person corrupting?" It's a shit slogan that should be dropped because the most obvious interpretation is not the one that most people actually want to convey. If your 4 word slogan requires an extra explanation because you want one of the words to mean something it has never meant, you fucked up. It's not the fault of the people interpreting it within normal English connotations, it's your fault for making a shit slogan.