For once i agree partially with the liberal, though obviously not for the nonsensical idealist argument that they try to make here. The role of police is to protect the state, enforce the existing order and safeguard the interests of the ruling class. Thus the blanket phrase ACAB can only be adopted as a universal slogan by anarchists who are simply opposed to the existence of the state.

For a communist the essential point is the class character of the state in question. In a capitalist society the state is the class instrument of the bourgeoisie, the role of the police as a whole is categorically reactionary and hostile toward the masses, thus ACAB applies regardless of the personal character of the individual cop. Whereas under socialism, which is the dictatorship of the proletariat, the state represents and serves the interests of the working class, thus the function of the police becomes the preservation of socialism, the suppression of the reaction, and defense against all counter-revolutionary forces.

And this is not just an academic, theoretical distinction by the way, it is very clearly evident in practice. Just compare the way the Chinese police behave and are viewed by the average Chinese citizen with how American or even European police treat and are perceived by most people there. It is clear there is a qualitative difference there in their relationship toward the masses, where in the capitalist case there is a fundamental hostility and mutual mistrust as police behave more like prison guards or an occupying army toward all but the class whose wealth they are tasked with protecting.