This is a bit outside the normal bailiwick of this blog, but is the sort of off-the-wall, half-baked idea that seems to fit here at least in that way.
Police work, at least as done in modern America, requires special authority, sometimes including the authority to use force in ways that wouldn't be allowed to a private citizen; sometimes the police make mistakes, and it is important to create systems that reduce the likelihood of that, but allowances also need to be made that they are human beings put in situations where they are likely to believe they lawfully have certain authority; if a police officer arrests an innocent man, the officer will face no legal repercussions, while a private citizen would, even if the private citizen had "reasonable cause" to suspect the victim. It is appropriate that this leeway be made, at least as for legal repercussions; if a particular police officer shows a pattern of making serious mistakes, even if they are clearly well-intended, it is just common sense that that officer should be directed to more suitable employment, but being an officer trying to carry out the job in good faith should be a legal defense to criminal charges.
That extra authority, though, comes — morally if not legally — with a special duty not to intentionally abuse it. This is the case not least because the task of police work is much more feasible where the citizens largely trust that an order appearing to come from a police officer is lawful than where they don't. A police officer in Alabama was reported, not long ago, to have sexually assaulted someone he had detained, and in a situation like that the initial crime is additional to the societal cost of eroding trust people have that the officer is at least trying to be on the side of law. This erosion of trust is also the primary reason that impersonating a police officer is a serious crime. I propose, then, upon the showing of mens rea in the commission of a serious crime by a police officer while using that office to facilitate the crime, that the officer be fired retroactively --- and brought up additionally on the impersonation charges.
 I mean, it should be. My impression is that it is too difficult to remove bad cops, but that's not an especially well-informed impression.
 Pressed to give secondary reasons, they would also line up pretty well between impersonating an officer and abusing the office.
 This policy would have an interesting relationship to the "no true Scotsman fallacy"; no true police officer would intentionally commit a heinous crime, and we'll redefine who was an officer when if we have to to make it true.