I don't know what they teach cops, but I know in the military you're taught that it's your duty to disobey commands that are unlawful, unethical or immoral.

If someone gives you an order to go smash some innocent person with a baton, what you're supposed to do is turn around and smash the person who gave you the order. This is the main check on abuse of state power - individual morals/ethic of the soldiers/cops on the grounds i.e. their ability to say "No. I'm not doing that."