The Model Penal Code developed in the 1970's, under duty to retreat, has two exceptions:
+When retreat would expose the attacked person to the lielihood of more harm
+When retreat would unreasonably take a person's freedom away, such as forcing one out of one's home.
I'll add a third one that is in CA law and other states, that can be called the "last aggressor" rule...if you start a fight, say, and then make it clear you are exiting, have no further aggression in mind, and the other party pursues you to reengage, that party has started a new fight and is the aggressor. Depending on circumstances, you can use lethal force if necessary in self-defense. One version of the Martin-Zimmermann situation is exactly this.
http://www.volokh.com/2012/04/03/the...y-and-liberty/
Good summary at the above cite.