I read the FL statute much differently then you, then:
"...initially provokes the use of force against himself or herself, unless:
"(a) Such force is so great that the person reasonably believes that he or she is in imminent danger of death or great bodily harm and that he or she has exhausted every reasonable means to escape such danger other than the use of force which is likely to cause death or great bodily harm to the assailant; or
"(b) In good faith, the person withdraws from physical contact with the assailant and indicates clearly to the assailant that he or she desires to withdraw and terminate the use of force, but the assailant continues or resumes the use of force."
Even if I rob you at gunpoint, if you then gain the upper hand and begin beating me to death, and I can't get away, I am justified in using deadly force.
Both parties are restricted to proportionate force.
At least that's my interpretation.
Unable to move with Martin on top of him, pounding him into hamburger (we still don't know how built Martin was -- how tall? how heavy? What did he bench-press in practice? etc.), Zimmermann might reasonably apprehend he was in danger of losing consciousness under his assailant's blows, and the assailant would then be able to disarm him and kill him with his own firearm.
Case of "use 'em or lose 'em" -- at the neighborhood scale. (Other end of the scale: General Thermonuclear Exchange.)