You're thinking of a different concept I think, i.e. when is it allowable to use deadly force at all.
If you fear for your life or are in danger of serious bodily harm you normally have the right to use deadly force (sometimes you must also have no way to escape, but let's just assume you're clear to use deadly force). Once you're in that situation it's advantageous to actually use that force. You wouldn't try to shoot someone in a hard-to-hit area like their knee or shoulder because as you say, if you had the time or perceived the danger to be low enough to line up for a shot like that, you were probably not in a situation so extreme that would warrant use of deadly force. It would be legally more defensible to go all Mozambique drill on somebody on put two in the chest and one in the head.
However, given that you are in the situation where you must defend yourself, but where deadly force would not be allowed, proportional response is usually still the operative concept.
If someone comes at you with a bat and you manage to disarm them that doesn't give you carte blanche in the U.S. to beat the attacker to within an inch of their life.
If you fear for your life or are in danger of serious bodily harm you normally have the right to use deadly force (sometimes you must also have no way to escape, but let's just assume you're clear to use deadly force). Once you're in that situation it's advantageous to actually use that force. You wouldn't try to shoot someone in a hard-to-hit area like their knee or shoulder because as you say, if you had the time or perceived the danger to be low enough to line up for a shot like that, you were probably not in a situation so extreme that would warrant use of deadly force. It would be legally more defensible to go all Mozambique drill on somebody on put two in the chest and one in the head.
However, given that you are in the situation where you must defend yourself, but where deadly force would not be allowed, proportional response is usually still the operative concept.
If someone comes at you with a bat and you manage to disarm them that doesn't give you carte blanche in the U.S. to beat the attacker to within an inch of their life.