Sorry, while I'll agree with most of what you said, I can't agree here. Police officers are putting their lives on the line. Someone resisting arrest has upped the likelihood of injury or death to the officer high enough that it may well warrant extreme behavior on the part of the officer.
That is not inherently the case. If I resist arrest by trying to punch the police officer in the face, by all means they should have the leeway to use necessary force. If I resist arrest simply by being uncooperative then no. I pose no risk.
We have to be very careful about authorising use of force outside cases where there's a clear danger to officer or public safety because it's not for the officer to punish you for resisting arrest, that's an extra item for the charge sheet.
But the sentiment that the cops were automatically out of line in these situations is troublesome.
The police wield state sanctioned force, a higher level of scrutiny is appropriate. Yes, there are questions about how much that should translate into liability for individual officers vs. the department as an institution, but every time the state engages in lethal, or near-lethal, force that is grounds to investigate.
Perhaps there's a cultural difference because, over here, if an officer so much as discharges a firearm there's a massive investigation; let alone if the police are involved (directly or otherwise) in a death. (Our situation isn't directly comparable as our officers don't routinely carry firearms, but I think the difference in culture is still worth noting. Also, FWIW, even when we didn't have particularly strict gun controls in the UK rank and file police officers were never routinely armed.)