I agree with you on one thing: the cops did not assess his breathing and failed in their duty of care once he was unconscious. As soon as he went limp, they should have turned him over and checked him, like the bystanders were telling them to do.

But, you could also argue that they did the best they could for what they suspected was a bad drug episode. I'm no expert but I could imagine that what they did was following protocol, and done for good reasons to try and save his life. Something like this seems reasonable:

1. Keep him restrained so he's not a danger to himself and people around him, until paramedics can arrive and sedate him. They will need you to have a handle on him when they get there, so don't let go when you already have him in a good position. The sooner he is sedated, the sooner he can be assessed and treated. Minutes count.
2. If he's drugged up, keep him down in a position so he doesn't choke on his own vomit.
3. Hold his head down, if he's delirious he might just smash his face into the ground. The cop would want to prevent that since it would also protect him from accusations of beating the suspect's face.

What bothers me most is that they didn't reassess anything once they observed that he was no longer moving. That's a huge failure, but I don't think it counts as murder: once he was unconscious the guy could've died anyway.

Above all, throughout this video you can see how the cops are very patient with him, and you can see that whatever happened was not because of malice or racism. It's sad that he died, but there is some gray area in what happened with regard to the police's intent. Unfortunately I don't think these cops can get a fair trial now with the way the whole situation was framed.