Good question, as usual with your scenarios.
The guy with the trash can lid was armed, with the lid, an item used or could be used as a dangerous weapon even though not a dangerous weapon per se like a knife or gun.
For an unarmed suspect, we had better be really careful what we do and how we do it because of disparity of force.
For instance, an unarmed man comes at you, wants a fistfight, you cannot just shoot him. Maybe he would be able to kill you, maybe he is armed, but right now when you shoot, you don't know any of that. Neither do we have to wait until we are dead ourselves. There is a fine line somewhere and it depends on the fact scenario presented. There are a couple of major rules and regs about it but they do not cover every conceivable scenario. Plus although self defense is mostly the same everywhere, your state may differ from others.
On the other hand, we cannot assume that an unarmed man knows the martial arts that can kill, either. We cannot say oh officer I shot him because I have heard that guys who know these arts can be deadly. You do not know this unarmed guy, so we cannot project what he may or may not do or what he is capable of.
We have to go beyond possibilities and guesswork. And it is not just if we feel our life in danger. We need a reasonable fear that we are about to experience death or severe bodily injury.
My general rule of thumb would be to never draw and fire unless you HAD to do it to save your life or the life of another. But even then, there are plenty of gray areas.
There are so many scenarios. Suppose you knew that a certain guy did have extensive training in the deadly martial arts and this particular guy is the one who says hey I am going to kill you and you know he can and will and he goes into a stance etc then that is a lot different from a guy who says hey let's fight and you say well I thought he might possibly know that martial arts stuff and so did not want to take any chances.
The various scenarios are what we must focus on. The general rules cover it but not all of it.
Reading that archived post, it seems clear to me that we cannot avoid a normal fistfight by shooting someone.
In that specific archived post you gave us the link to, it is clear to me that the guy with the gun would have been charged had he pulled his gun out and fired, when the other guy was delivering a punch. That is disparity of force.
A fist vs a gun?
A stick vs a gun?
A baseball bat vs a gun?
A knife vs a gun?
Now what you have is increasing force being used against you, and so the correctness of using your gun becomes greater and greater. If you have knowledge that something else is operating, though, like the guy swings at you but puts his hand on a knife and it is still in a sheath and he opens it and now has it in his hand, his chances of continuing to live have just plummetted.
What force is necessary to repel my attacker? Use just that force necessary, and no more. If you use more, YOU become the aggressor and thereby lose the right of self-defense because you exceeded the lawful bounds of self-defense. Self defense is not unlimited.