In law the cop is responsible, at very least reckless endangerment. It is unlawful in UK to place a person face down and use force to keep them down, except in a spontaneous action in self defence. Once you have control, a handcuffed man is under control, you must lift the person up from the face down position. As a cop you are fully in charge and have an absolute duty of care for the person.
If said person is a threat, for example he could be spitting in the officers face, you do have leeway to hold them down.
The law is such, in UK, that the cop or a cop, MUST take action to ensure a face down person, no longer a threat, must be lifted up. The law says you MUST lift them up. Not that you can choose to keep them down.
This cop had his hands in his pockets while kneeling on Floyds neck, thus his whole weight will be felt by Floyd as impact on his neck.
That Floyd took drugs is only relevant if he would die anyway even if the cop took his knee off after a few seconds.. the drugs killed him. So death, murder, is not the factor.
The cop had a duty of care and failed to exercise his duty of care. Negligence being the action/causal affect.
You can omit/fail to do, or do something which was the primary cause. The cop was right to restrain Floyd who was recalcitrant and a known very violent man, but the duty of the cop includes accountability for his actions/failure to act. He failed to allow Floyd to sit up. Would Floyd have died even if he was sat up due to drugs and his refusal to cooperate with police?? Floyd also has responsibility for his behaviour and criminal behaviour does not get sympathy (except from leftie pretend lovers of the BLM).
Overall, I think the cop was wrong. The crux of this case will fall on whether Floyd could not breathe due to the cops knee or could not breathe due to taking drugs.