Once Floyd was no longer resisting there was no reason to keep beating him up. While his misbehavior (possibly drug induced) would allow the police to subdue him, it would be for the judge to decide what to do with him (note that American law no longer allows beating up people as punishment). While there were elements of coercion used on the jury (letting them know that a wrong verdict would lead to massive violence, which implied their lives were at risk if they didn’t give a politically correct verdict), based on the press accounts it is reasonable that a jury could find that if the officer stopped using force at the point that Floyd was subdued and handcuffed, Floyd would not have died that night.
If a person was authorized by Beis Din to subdue an insane person or a criminal, and after the person had been subdued, the person sent by Beis Din decided to beat him up (note that the Beis Din had not decided to order the person flogged, which they wouldn’t do if his misbehavior was due to drug overdose), and as a result of the illegal physical punishment (beyond what was necessary to subdue the offender) the person being beaten died, the person sent by the Beis Din would be in very big trouble. Note that when a Beis Din ordered someone flogged, they had to confirm that they weren’t killing him.