Police are necessary because it’s an imperfect world and always will be. There will always be a need for some functions that only a police type organization can furnish. Even the most strictly policed places, prisons and military bases have those problems. Yes, prisons are full of people with behavior problems so let’s deal with that separately. The military, however, is composed of motivated basically normal people, still disagreements and misunderstandings will occur and those can require a disciplinary means AKA police.