In Minnesota third degree murder does not require intent. Only "perpetrating an act eminently dangerous to others and evincing a depraved mind, without regard for human life..." It's not hard making that case in this incident.
Hmmm. We shall see. You do understand that the act of putting a knee on the back of the neck is trained technique in the Minneapolis police department right? So him doing that was part of his training. you may disagree on why he did it or that he needed to do it for how long he kept it going, but the act itself is a legit police technique in MN.