They weren't scratches, either. So, what would his injuries have been if he hadn't stopped Martin by shooting him?
That's not the question to be answered in a justifiable use of force defense. Is it reasonable to believe, had he not used deadly force, he would have suffered severe bodily injury or death?
A broken nose and lacerations to the back of your head with any disorientation resulting from having your head assaulted might lead someone to reasonably believe that.
Zimmerman took a gun everywhere.
Following isn't chasing. And, there's no evidence he instigated or chased.
When did Zimmerman catch Martin? He had lost sight of him by about halfway through the conversation with police and it went on for another 2 minutes or so after that.
Only if it were supported by the evidence.