Shooting someone who is no longer posing a threat to you is not self-defense.
This man, being an officer, had arrest authority, and should have used it.
They only ran off after he drew the gun. That means before he pulled out his gun, they were still beating him. Letting them run off was a dereliction of duty. If some people break into your house, assault your wife and children, but start running when they see you entering the room with your pump shotgun, would you just let them run off?
Yeah Dave, but some folks just need shootin’.