So, are you saying Zimmerman had just legal cause to shoot?
If Zimmermans account of the attack is correct, that Trayvon attacked him, then yes he had the legal right to use force. He was on the ground knocked there by Trayvon, being beaten and Trayvon was trying to forcefully take his weapon, that is grounds for killing.
if he was in fact being beaten to a pulp with an already broken knows etc., then yes.
it would be analogous to a woman being raped stopping her unarmed attacker.