Nothing silly about lethal force, and when it is justified. My logic is predicated on the definitions of threat, and of necessity, in the use of lethal force.
Once Mr. Ahmed wrestled the rifle away, the shooter was disarmed - and hee chose to start walking away - and had Mr. Ahmed fired the rifle, he would have had to shoot an unarmed man in the back.
He was no longer a threat.
His breathing has nothing to do with it, and I wish Australia still had the death penalty, but the punishment for his crime belongs with their courts, not a summary execution after he has been disarmed.
I would encourage you to talk with a defense attorney, or read one of many books on self defense, like those by Massad Ayoob, to understand the principles I have raised.