Being able to justify using deadly force against an unarmed person is not hard. Let's look at this simple scenario. Fyi, I am an AACFI certified Firearms Instructor. Those of you who have taken an AACFI course should recognize this.
It is late at night and you just ran out of gas in a bad looking neighborhood. You now have to walk a mile or so to a gas station you think you passed. You are carrying concealed. As you are walking you see a man start crossing the street heading straight for you from let's say 50 ft away. As he gets closer, at say 40 ft he asks if you have a light. You reply that you don't smoke. He continues to approach anyway, saying something like "come on man, help me out" or whatever. At 30ft you subtly place your hand on your weapon. You do this because now he has broken the distance established by the Tueller drill. (for those of you who don't know what that is you really need to look it up) He continues to approach saying whatever at this point, it really doesn't matter because he has no reason to approach you other than to do you harm. At 20ft you assume a more defensive posture, put you hand out and say STOP BACK OFF or other things to that effect. He continues towards you and around 15ft you draw your firearm and present it still shouting commands at him to stay away. You also begin to retreat, creating distance between you and your attacker. He still continues to advance and about 10ft away you discharge your firearm. Let's skip the aftermath other than to say that he was found to be unarmed. Here's how this was a justified shooting.
You established that you were a reluctant participant. You did not willingly go into that neighborhood, nor did you at any time show that you wanted to be in that situation. In short you did not seek out that conflict and it found you anyway.
You established that you were in imminent fear of death or great bodily harm. This fear was due to the fact that he continued advancing for no reason. This implies evil intent. The fact that even after you warned him off and presented a firearm he continued to advance on you. This implies that he is either insane or assuming that your bluffing and won't shoot him because he appears unarmed. The fact that if you let him get upon you that you would now have to retain your firearm and fight him off with only one hand. The fact that he could have easily been armed and just chose not to pull something out until he was closer, lulling you into a false sense of security. Remember every conflict you are in is an armed conflict even if the other guy is unarmed. That firearm can be taken from you and used against you if you are not able to retain it. This also helps to establish the next criteria.
You established that no lesser force would due. You had no other weapons other than your hands/feet and your firearm. Since you were not confident that you could retain your firearm and fend off the attacker one-handed you only had one other option.
You established that retreat was not practical. You tried to retreat and he continued to advance upon you. Turning and running was not a valid option because you do not know if he has a gun or not and could just easily shoot you in the back. Certainly his behavior implied that he might be armed. He was continuing to advance upon you even after you presented your firearm.
Under these circumstances your shooting was justified.
Now we can play the "what if" game until the end of time but if you meet those four criteria then you are justified. Please try and remember that his is just an example and was designed to show you how you can justify those four criteria.