> what those perps and officers have experienced
Both totally irrelevant to whether or not it's appropriate to punch a handcuffed prisoner in the face while they're not resisting.
If a cop is suffering from PTSD or stress to the point where they can't keep themselves from assaulting a handcuffed prisoner, then I am genuinely very sorry for them, but they're still in the wrong job and they still need to be let go.
Treatment combined with appropriate work should be the first option. Treatment combined with sick leave should obviously be the second.