As I see it, what the jury will have to decide is if the shooter commit a very bad mistake, or a crime.
The video shows the attack had ceased, BUT it video wasn't taken through the eyes of the guy on the ground, who just got knocked down, and cracked his head on the pavement.
It is possible that the shooter did not recognize that the attacker had backed off,. It also possible that the shooter did recognize and shot him anyway.
Another point, and one which could change all our speculation is what was said before and during the attack. We don't know what that was, and without knowing, about anything is possible. Suppose the woman had been telling him that "her boyfriend will kill you when he gets here", or something like that. Then the guy gets blindsided. Might that create a perception of a real, serious, and imminent threat?
Whether it was nothing, or the deadliest threat imaginable, anything could have been said, and we simply don't know what, but it could have been a factor, one we cannot account for at this time, if ever.
Just playing devil's advocate here, but if I had just been knocked down, maybe wacked my head, my vision might not be as perfect as usual for a second or two maybe a little blurry, and I might "see" the guy who just assaulted me "looming" over me, or moving toward me (and therefore a threat) when a view from a camera at a different angle shows something else. We're talking about a very short period of time, what the shooter perceived as reality, and what reality actually was. It's not as simple as it seems at a glance.