From The Star King by Jack Vance:
Humanity many times has had sad experience with superpowerful police forces… As soon as (the police) slip out from under the firm thumb of a suspicious local tribune, they become arbitrary, merciless, a law unto themselves. They think no more of justice, but only of establishing themselves as a privileged and envied elite. They mistake the attitude of natural caution and uncertainty as admiration and respect, and presently they start to swagger back and forth, jingling their weapons in a megalomaniac euphoria. People thereupon become not masters, but servants. Such a police force becomes merely an aggregate of uniformed criminals, the more baneful in that their position is unchallenged and sanctioned by law. The police mentality cannot regard a human being as other than an item or object to be processed as expeditiously as possible. Public convenience or dignity means nothing; police prerogatives assume the status of divine law. Submissiveness is demanded. If a police officer kills a civilian, it is a regrettable circumstance: The officer was possibly overzealous. If a civilian kills a police officer, all Hell breaks loose. The police foam at the mouth. All business comes to a standstill until the perpetrator of this most dastardly act is found out. Inevitably, when apprehended, he is beaten or otherwise tortured for his intolerable presumption. The police complain they cannot function efficiently, that criminals escape them. Better a hundred unchecked criminals than the despotism of one unbridled police force.
In other words, more centralized control over local police forces
is a terrible idea.