None of the shots hit him from behind. All three autopsies agree on this. The rest of that quote is just noise.
8+ shots fired rapidly in two bursts within 10 seconds doesn't not indicate much for either side. In defensive situations, you fire until the threat is stopped. I don't think at any point the cop ever thought "I'm going to kill this kid". I think he reacted to a situation that escalated quickly into a life or death situation much the same as any person would. Maybe he overreached, but maybe he used the exactly the amount of force needed to resolve the situation. I simply don't have enough information to know and I doubt anyone getting their information from media reports does either.
If 5 seconds ago, someone attacks you and tries to take your weapon, yes, the threshold for justification of lethal force is crossed. In fact, once the altercation escalates to the point of struggling for a cop's weapon, the only way to deescalate it back to a non-lethal level is complete and unconditional surrender. The cop has every right to attempt to stop a fleeing attacker even if lethal force is required.