I would say that he was cuffed and there was no reason he had to keep his knee on his neck. The man could of died from a drug overdoes, he still doesn’t excuse the fact that they could of had him sit up with his back to a police car.
Does that relieve the officer of his responsibility in this. Probably not...however the guy was having difficulty standing, sitting and walking without assistance from the get go.
I think we're going to find he was likely intoxicated/drugged , belligerent and incoherent. As for the officer's conduct....that remains a question if or not he went too far in securing the guy. But he certainly wasn't there to deliberately murder anyone.