@inthehands If you're talking about US-model policing, one starting point might be the Peelian Principles. Mostly fallen by the wayside in the UK after 200 years but they were revolutionary in the 1820s, and to this day British cops aren't as venal and violent as their US counterparts.
Take the Principles and add an overriding duty to "protect the public" as higher priority than "enforce the law"—bearing in mind that even suspected lawbreakers are part of the public—and it's a start.