Police are hyper-violent because they have no real purpose.
What if the reason why the police have to act hyper violent is because they don't have any real necessity and thusly, no real authority. In Foucautian sense, the State engrains a self-policing in its management of the denizen's body, to the point where the State can regulate your health, can know where you are with tracking chips in your passports, CIA surveillance etc. You know they are watching. Once you know, you are subject to the panopticon.
From Wikipedia: The Panopticon is a type of institutional building designed by the English philosopher and social theorist Jeremy Bentham in the late 18th century. The concept of the design is to allow a single watchman to observe (-opticon) all (pan-) inmates of an institution without the inmates being able to tell whether or not they are being watched. Although it is physically impossible for the single watchman to observe all cells at once, the fact that the inmates cannot know when they are being watched means that all inmates must act as though they are watched at all times, effectively controlling their own behaviour constantly. The name is also a reference to Panoptes from Greek mythology; he was a giant with a hundred eyes and thus was known to be a very effective watchman.
Once people are ingrained with the self-policing of the panoptic by the State, actual police power is ultimately subverted. In this sense, so is the seeking of power that makes one self select as a police officer, that causes one to live within the reach of death as a person who carries a gun at all times. (Once you have a gun, all acts of defiance must be executed with deadly force because the person with the gun has authority of deadly force.) In this way, the power seeker cannot be fulfilled because a modern policeman's actual role is not to enact power but to serve as a representation of power, like a cop at a baseball game. No one expects that cop to enact his power, he or she is there just as a show of force so people do not act up. If the police officer himself expects his job to be standing around and driving around as a symbol he will be satisfied. But, if said cop expects to be enthralled in action, as an urban solider, he will be greatly dissatisfied, since, in light of the state he is not needed for this purpose. This then creates a self fulfilling prophecy where the police officer sees himself as an urban soldier and then uses excessive force in all interactions as he acts how he feels he should and not as he is needed by the system. If policemen become policeman in order to gain power in a system that promotes panopticonian self policing, these police are doubly dissatisfied because they must also realize that they have no actual power because they have no actual purpose.