Tired of hearing "the police aren't supposed to..." Who says? The police are supposed to uphold the capitalist, white-supremacist, patriarchal order through violence. If that means arresting and killing innocent people, they'll do it. It's meaningless to say they're not supposed to or they shouldn't. It's their job. The point is, their jobs should not exist.
When you're considering a social institution, don't go by someone else's words. Look at what it does and the results of its actions. Only then will you know its purpose. Words on paper only tell you what their PR people have been paid to tell you.