Warning shots are for people who want to be killed.
They signify you aren’t prepared to take a life.
Crooks will assume you are bluffing and not serious.
Here’s a thought, most places where self defense is legal, to justify your use of lethal force, you must be in eminent danger of death or serious bodily injury. If you are not in mortal danger, why are you using lethal force, I.e., the gun, when that level of force is not justified and is shown by your actions, that warning shot.
You have to shoot, shoot to stop, if they die because of their actions, too bad for them.
As to 74’s scenario, it would depend. Are these Nat’l Guard or Police looking for survivors? Are they obviously looters? Did they just kill the nice little old lady down the street?
Untrained? 5:1 odds, they may be in trouble. I can get at least two before the others can react, if not more. At what distance? Thats gonna depend on the weapons involved.
Firefight or gunfight? I hope not. I’d prefer to be involved in a shooting than a gunfight. In a shooting the bullets only go one way.
As to the moral and legal issues, study your local and state laws to know when you’re legal. Only you can decide on the moral implications.