Pause this it at 1:03 in this video. Notice that the tackler's torso is not facing the ball carrier. It's actually facing the ground, not the ball carrier. It's facing away from the ball carrier.
Yet obviously he still made the tackle, because regardless of the fact that he wasn't facing the ball carrier, his momentum carried him into the ball carrier.
Understand now? Or no?
Mother of God you're reaching, and you're failing.
First, off, we're talking about someone being shot in the back, not being tyackled in a football game. Second, if the tackler's back had been facing the ball carrier, he never would've made the tackle. He would've been no threat. That would've been an appropriate comparison. But, that comparison only proves you wrong, so you want nothing to do with it...
Rosenbaum was charging at Kyle just like that football player.
Also, someone's back doesn't need to be facing you in order for you shoot them in the back, jackass.
To prove this to yourself, stand several feet way from your kitchen table. Imagine that the top surface of your tabletop is the back of an attacker who is rushing towards you.
The attacker is not facing you, it's facing away from you towards the floor and it's back is facing away from you towards the ceiling. Now see if you can shoot a picture of the table top with your Cannon.
If you can, that's proof that someone does not need to be facing you to be a threat, and you could shoot that threat in the back.
Anyone who thinks that a gunshot wound to the back constitutes evidence that the shooting wasn't in self-defense is a fucking moron.