Any one who watches the full video sees Floyd is having major issues breathing long before he gets knelt on. His lungs were full of fluid, more than enough to kill him. The cops also called the ambulance about that time. He requested he be put on the ground instead of the car.

Should that cop have knelt on him? It's a dangerous restraint method and it shouldn't have been taught, and it was legal. But he thought Floyd was faking the shortness of breath to get out of arrest... He was a burly man known to physically resist arrest... Judgement call.

Should he have continued kneeling on him so long after he was clearly unconscious? No.

Did the cop kneeling on him kill him? Impossible to say, as he was quite probably (not certainly) going to die imminently from the fentanyl induced edema.

Does this reach the legal requirements for a murder charge? No, because you need to be sure that the kneeling caused his death. We can't even be sure it contributed to Floyd's death, the edema could easily have killed him if he was gone alone on the couch with no neck kneeling.

So so so kind of excessive force charge is probably what they'll settle on eventually.