It’s a close call. Thug assaults victim, then continues to approach victim, until victim pulls out gun. Thug, seeing the gun, starts to back up and is *arguably* no longer a threat. Victim shoots thug anyway. It could go either way.
I don’t know if the thug threatened the guy on the ground or not after the gun was pulled. He might have told the shooter that he was going to kill him or cause some other harm. Any verbal threat at that time could cause the shooter to fear for his safety of life because of the shove to the ground (unprovoked assault) he had just been the victim of.