If you're in a hand to hand struggle with someone, you can tell when they go limp.
And people were shouting at him "Stop you're killing him" (approximately) and begging him to get off the guy's neck and take a pulse.
That's specific enough intent for me.
Particularly as, if the man was handcuffed, then they are legally responsible for his safety at that point. Custody, therefore Custodian.
What if he was actually dying from a drug/alcohol overdose?