McGlockton was not retreating. He had just advanced two steps after a previous retreat and he hesitated as the gun came out. Then he side-stepped. That was not an "end of the threat".
Meanwhile the girlfriend was making a rapid move at the edge of view. Was she coming to assist in an attack? It would be a "reasonable" assumption under the circumstances.
Drejka was down and could not retreat.
Drejka had (IMO) "reasonable" basis for believing that he was still under threat of imminent death or crippling injury.
McGlockton was still the aggressor in this tableau and was bigger, younger, and stronger than the shooter. That establishes the "disparity of force" that defines a lethal threat.
Drejka fired only one shot at a man who was aggressively facing him as a potentially lethal threat. That establishes "proportionality of response".
The racial component of this incident is the only thing that makes it controversial. If it had been two Blacks or two Whites involved, the discussion would have ended pretty quickly in favor of the shooter.
If the shooter had been Black and the attacker had been White, the shooter would be getting (undeserved) praises as a hero of some kind.
Note: I believe the shooter was probably being an officious jerk. That still does not justify knocking him down and moving to stomp him. And it does not impair the justification of a lethal self-defense shooting.
I guess you missed where he took two steps back ...