True, thought the shooter would have several things to explain, most likely to a criminal court...An interesting concept could be where a person, armed with a firearm, is confronted by another person who is unarmed but physically capable of and determined to attack the armed individual and take their firearm to use against them and/or others. Shooting this person could be justified as not to have done so could reasonably have resulted in the death of the person being attacked and that of others unable to defend themselves against the attacker once they had obtained the weapon.
Why were you carrying said weapon? Why was it loaded? Why did you confront this person? What other options did you consider? How could you have possibly known so much about this person's ability and intent to use lethal force against you, using your own weapon? Are you lawfully allowed to carry loaded firearms? If you knew so much, why did you put yourself in this situation in the first place? And so on.
People have shot and killed other people in Australia and not been charged with an unlawful killing offence, but it's an exception to the rule.
A self-defence argument is always going to be examined in minute detail in the case of what otherwise, would be a homicide (even if it is only manslaughter through diminished criminal responsibility).