This was a very interesting question asked by one of my readers, Jim Hancock. He actually surprised me by posing this question. I had never been asked this kind of question. After considerable thought, I found that this question and my answer would serve all by writing an article about the issue.
I knew the answer beforehand but decided to spend a few days researching if this has ever happened in the United States or anywhere else. It has happened in the United States and in Europe. I also decided to research a few state and federal laws pertaining to this intriguing question. The answer is what I first surmised, yes!
An example:
If you and your family were relaxing at a park, and saw a police officer or any law officer stop an innocent individual (or one of your family members at the park), handcuffed him or her, and then for no "obvious" reason begin to beat the handcuffed person mercilessly, potentially causing "great bodily harm or even death, you could intervene, but expect the legal system to examine your case in detail!
And if while you and your family intervened, the officer begin to pull his revolver on you and your family, and then you shoot and kill him or her, you could claim self defense, with a qualification!
The qualification: You and your family, or you and other witnesses, must have witnessed the violation! The key word is witnesses. You better have others that are willing to testify how the event occured!
A police officer, fairly or unfairly, is almost always given the benefit of the doubt. To have witnesses on the scene "seals" your self defense argument. Police officers are not above the law, they are not the law, they are only commissioned to "enforce" the laws. He cannot violate laws and ordinances, and if proven or witnessed to have the broke the laws, he is subject to the same repercussions as any citizen. But you better make sure that you have at least another witness,or else. The police chief and prosecutors will attempt to prove you wrong.
As I've mentioned in many articles, police departments habitually side with the officers most times, and prosecutors "break bread" with police officers daily. In other words, they tend to side with each other.
I can remember while an officer in the 70's consulting with district attorneys on many occasions where the district attorney would tell me what to say or do on a witness stand " without actually telling me what to say"! You know what I mean? A certain wink, gesture, or in one case, actually writing down on a piece of paper what I should say and discarding and distroyng the incriminating document. I didn't have the guts then to report these rogue attorneys. And guess what, many officers today, almost all officers, won't report them either. With many police officers and some district attorneys, everything revolves around "covering your backside".
In fact, the only times I've seen another officer go against another officer during my tenure was when an officer feared implication by another officer. It's when he feels another officer "ratted" him out or some similar issue.
Getting back to the subject, know this. It's your duty as a human and citizen, to protect your fellow citizen. Just know beforehand that if you engage a rogue cop while protecting yourself, your family, or other strangers, you must quickly deduce the ramification of your actions and always try to respond in pairs or with other witnesses. Thanks Jim for a very intriguing question.
(Disclaimer: I'm only giving my opinion to a question asked of me by a concern citizen, and I'm not advocating ever getting involve in this unique situation).