Well, if they saw him shot from behind, it matters not whether his hands were in the air. Obviously he was still fleeing, not surrendering, since he didn’t turn around. That’s resisting arrest, after assaulting an officer among other things.
More than resisting arrest. In most states, a perp who assaults an officer, has now become a threat to public safety and MUST be pursued and taken into custody,...to protect the public.
I suspect the officer didn’t have a choice to legitimately ignore Brown’s actions, nor could he allow Brown to walk away. Either Brown submitted to police authority and had to be taken into custody or physically halted from leaving the scene.