Fucktons of autism in this thread, and I'm pretty sure there are a couple of LEO posters in the mix.
The sperging armchair quarterbacking about restraint tactics is kind of exceptional, and I guess I'll join it. Handcuffed and three guys still subduing not a threat? I wouldn't say so. IF a cop is restraining someone in the line of their duty, I want them to have complete control, if not willing to get it, then they shouldn't even attempt the arrest. Drugged, small hands, broken cuffs aside, they pose a threat to not just the cops, but themselves and bystanders. Handcuffed on your belly might look like you're incapacitated, but you're really not.
A knee to the back of the neck is pretty painful and incapacitating, though, and every cop would know it. I've had it happen to me a couple of times in the past, and, after the cops secure things, I've been taken out of handcuffs, had a few laughs, and shot the shit with them before giving a statement about whatever they were there for and sent merrily on my way. It's a shit job I don't envy them it. Of course, being polite and not resisting is the reason I'm not a statistic like this dude. Don't think it has anything to do with race.
Doesn't excuse this cops behaviour, though. Maybe just a bad day, or he was wound up, or whatever, but he fucked up. Murder? I doubt it. It would be good to see his record. Something tells me it's pretty good. Should get a negligent homicide charge for sure, IMO (and assuming the autopsy shows it warranted), and if his history is at all dubious, he should likely face significant prison time. The other three? Not so much. Some internal financial penalty and some administrative punishment, maybe spending the rest of their careers doing community outreach at schools where a bunch of little shits call them faggots or whatever.
I'm a bit more concerned about the state of their police force, when command is willing to fully throw them under the bus before any sort of audit of the incident. No good for police morale, but even worse for honest citizens, with a demoralized police, undoubtedly now less willing to protect, and undoubtedly more prone to corruption and ignoring legitimate arrests as well.
The one cop fucked up badly, maybe through malice (maybe we'll find out maybe we won't), but police command's response has caused even more dmamage to the city they are supposed to protect than the cop's fuck up.