This is not the kind of situation in which a so-called "Good Samaritan" law applies. Such laws are for protecting people who aid accident victims, usually by giving first aid, but also by helping them out of dangerous situations. "Good Samaritan" laws prevent the accident victim suing the helper when things don't turn out well, although there are limitations on such protection.
The situation described in the question is "defense of another" which is generally covered under the same laws as self-defense. The exact laws vary by state, but in general when one person is committing or threatening unlawful violence on a second person, a third person may use reasonable force to protect the victim.
Only "reasonable" force, normally just enough to stop the attack or protest the victim, is allowed. The judgement is based on what a "reasonable person" would have done in the same circumstances. The seriousness and danger of the attack would also be considered.
For example, deadly force would not be permitted tom stop a person giving another a bloody nose.
In the situation described in the question, it would depend if the person had a plausible way to defend the officer or stop the attacker without using deadly force, that is, force likely to kill.
A Chokehold is inherently dangerous, and can result in death even when that is in no way intended. The authorities would (or should) look at the facts of the case and what other options the defender had. There is no automatic protection for the use of excessive force.
However, a murder charge seems unlikely, as the question does not describe an intent to kill. Manslaughter might be more likely, but the details will matter a lot.
If the prosecutor thinks the defender had no other reasonable choice, it might well be that no charges would be brought. If they were brought but a jury thinks the defender had no other choice, there would be an acquittal.