If a person isn't giving consideration for the potentiality of the perceived threat he is responding to, how can he say whether his act of deadly force is reasonable?
It depends on the circumstances. If the perceived threat is outside, in broad daylight, and the perceived threat is a gun, then the attacker should have something that looks like a gun. If the threat is fists, then the attacker wouldn't be holding anything.
If the threat is at night, and you cannot tell what the person has, but they're on you're property, then it doesn't matter whether you think they have a gun, knife, bat, etc. or just their fists. It's the conduct that matters (for example, trying to open someone's door).