I completely agree with you on ACAB in capitalist countries, for the same reasons you mentioned, but cops in "actually existing socialist" countries like Russian and China are no better. They still use authoritarian violence to oppress anyone who steps out of line with the will of the State. There are many, many historic and more contemporary examples of socialist countries using the [secret] police and/or troops to quell dissent from unions, anarchists, and other leftist groups, because anyone who protests the actions of the State, no matter how legitimately, is considered to be an enemy of the State, whether that State is capitalist or not.