The BG was moving away and clearly so when the shot was fired. If the killer just had shot in the air that would have made the deceased run even faster.
This was a destructive moment and will have political consequences, not the least firing up the gun grabbers.
Agree this is not a clear case from the viewpoint of the video. However the law states if the defender had a fear of imminent deadly force attack. Clearly the deadly force attack bar had already been met. The question is was the shooter in imminent danger of that attack continuing and I would say the answer is clearly yes. Police are very routinely exonerated over a shooting of an attacker based on the "21 foot" rule or Tueller rule. The attacker was definitely inside 21 feet and even though it appeared he backed up slightly he could have simply been making a move to circle the victim and continue attacking from a different angle. I think a good lawyer will bring up the 21 foot defense very quickly. In my opinion this is a clear case of self defense.