I've participated in law enforcement hand to hand and incapacitation training sessions (as a Marine). At least in those trainings, it was very clearly taught what techniques were safe and effective, how to use them, and we practiced them on each other literally hundreds of times. If you've ever had a blood choke or constrictive choke applied to you, you immediately understand what it's like, and what dangers it presents. The officer charged knew what he was doing, without a doubt. The officers around him had a moral and legal requirement to stop him to avoid being complicit in this murder; they chose to help in the murder instead.

In the military, I would have been convicted of a war crime had I done this to an unarmed and incapacitated enemy combatant; even in ignorance, even if it had been a mistake, even if I was tired and had a bad day. I'd hope our police force would be held to a higher standard dealing with our own citizens than a soldier or Marine dealing with an enemy of the state.