The traditional requirement to avoid lethal force served us pretty well for a hundred years or so. what was wrong with that?
Did it? I don't know whether that is the case or not. Avoid lethal force: (1) completely?; (2) unless you reasonably fear that you will die if you don't use lethal force?; (3) unless you reasonably fear that you will die if you don't use lethal force AND something else -- you did not (a) cause; (b) provoke; or (c) escalate the situation?
I suspect these legal standards have always been controversial, especially since they deal with matters of life and death.