The police generally consider laws to be for little people. In the US, any law that is supposed to apply to police has to not depend on the District Attorney ever filing charges, because they won't, except for certain high-visibility murders, and usually not even then.
Something that has worked sometimes is a state or federal law affecting city police. The law should have provisions to allow citizens to initiate enforcement, and, for egregious offenses, provisions to restrict the convicted officer from working in other police departments.
Too often, departments appear to respond to public pressure by firing the officer. But that actually achieves very little; the officer just goes to work for another city, with a fresh, clean record. There are a lot of cities.
It would be helpful to establish a national registry of police behavior, so that offenses at one department follow them to the next.