Police in genre fiction have an interesting history. For a long time held as paragons of virtue, the cop was an easy symbol of virtuous authority. Even films as recent as Mad Max (1979) and Robocop (1987) held that the police were an ultimate force for good. That has changed, especially within the last forty years, where films began to ask us to question who was holding those positions of power; specifically, films like Minority Report (2002) and A Scanner Darkly (2006) dispel the notion of the virtuous good for police to create a more complex character that stood in shades of gray.
Oddly though, this idea of the police as being good or bad side-steps a much larger issue with the representation of police in popular fiction: the very existence of police in the first place. As with the real world, police in film don't exist to prevent crime or even, really, to cause it; they're function is to preserve power systems simply by virtue of their presence. The symbol of the police officer is more powerful than the man (or woman) inhabiting the costume. In film, this stands a clear delineation of values through the coded image of a person in uniform. We immediately understand there is a right and wrong decision to be made because that person exists before us certifying it. Either the officer is doing his job, upholding the law, or he's not, he's breaking it; regardless of which it is, there's an explicit moral point underscoring his role that's arguing his job as an arbiter of the law is fundamentally right and by extension the society that shapes those laws is fundamentally right. If he's a corrupt cop, that isn't an indictment on the profession or the laws that created it, it's an indictment on him as a person -- he's done something that's gone astray from society. It's not an authority issue, necessarily, but rather one of being.
A clear example of this is the 2012 film Dredd. Structured as a day-in-the-life of a police officer living in a dystopian future, Dredd follows the title protagonist as he grades a rookie during her field-exam day. Throughout the course of the film Judge Dredd (all officers are known as Judges, as they exist as judge, jury, and executioner) exists at least somewhat as a self-conscious exploration of the character as fascist figure. The film never questions the role of the Judge in this society even when a major plot point hinges on corruption; Dredd himself, and his student Judge Anderson, are still characters of virtue. In fact, Dredd himself is presented as an absolute -- within the context of the society he exists in, he’s an unbreakable pillar of moral virtue. This stands at least somewhat in contrast to Dredd’s source material where the Judge evolves into a more complex figure who eventually questions his presence in society. More broadly, this is also in contrast to a slate of genre films which question the very existence of police in society.
Culled from the mind of writer Phillip K. Dick, films like Minority Report (2002) and A Scanner Darkly (2006) examine how the existence of police in society shape our interactions with everyday life. In Minority Report, a “precrime” branch of law enforcement has developed which can see future crimes and punish individuals before they happen, while in A Scanner Darkly a world exists in which almost everyone has been deputized with the intent of maintaining a surveillance apparatus in which there are no civilians because everyone is spying on everyone else.
In contrast to Dredd, these two films argue that the very existence of a policing body may influence interpersonal interactions to the extent that it creates crime, consciously or otherwise, as a way of perpetuating the need for it in society. In Minority Report, free will evaporates in the face of a crime-fighting entity that can predict human behavior, so we’re left with the pre-cogs to save us from ourselves. Meanwhile, A Scanner Darkly posits that we’re all unwitting pawns being used by a security state to fulfill its own needs in perpetuity, taking from us the ability to even exist as individuals.
Unfortunately, genre films dealing with police tend to fall more on Dredd's side of perspective rather than the views of a writer like Dick. Because of this, our perception of police returns to the issue of authority instead of being. We question whether their power is legitimate, ultimately certifying that in some instances it must be, rather than asking if they should even exist in these forms at all.