By that logic, if someone got into a fight and got punched in the face, he could consider the other person as "trying to kill him". The face is above the neck, after all. He could be knocked unconscious and smack his head on something on the way down. He could suffer a concussion or permanent brain injury. Could he then justifiably produce a gun and shoot the other person to prevent these possible life-endangering outcomes?
Besides, you're ignoring that Zimmerman didn't use his gun to stop the beating. He used it because Martin was going for it. If Zimmerman himself didn't consider the fight life-threatening until the gun came into the picture, why make that assumption for him?