Is any organization or individual attorney standing up for this guy? Clearly this was unintentional and a measured response to aggressive action. I suspect the cause of death was an underlying condition.
Probably not. The problem is more likely an accident of training: Marines are not trained to restrain in combat - hand-to-hand combat as trained in Boot Camp and after is to overcome the enemy using that hold that he used and others.
I am sure that no one in that training ever told those recruits to use those holds on civilians, just enemy combatants, so there wouldn't have been any emphasis on watching to see if your adversary was unconscious.
That training makes a hold like that automatic. He didn't use the version where you place your left edge of left hand just past the enemy's chin and your right hand over the top of your left hand and then throw your feet straight back, putting all of your weight into the pressure on his upper neck. That was called the "Kill Hold" and was meant to break your opponent's neck. He didn't use that one.