I tend to agree that it’s very bad practice to ever threaten with a gun. If you have a gun and think you are in danger for your life and it’s the only way to protect yourself, you shoot. If you have a gun, but do not think your life is in danger, you do other stuff–call 911 and lock your bedroom door, go out the back door, whatever.
But there’s no point in threatening with a gun. It’s pointless escalation, because now the other person is afraid they might die, and scared people do stupid things.
If someone is genuinely in a corner and has no way of knowing what’s going on (like they wake up with a stranger in their bedroom, for example), I don’t think they have to warn anyone. But if someone is in their own bedroom, hears someone downstairs, gets out their gun and goes to confront them instead of calling 911, and then shoots them after a warning, that’s wrong. It never had to come to that.