Exactly.
There are two basic scenarios that could've happened, with 3 judements. Either it was like Zimmerman said and he was out of his car finding an address and trying to see where Martin went, lost track of him, then got jumped while he was going back to his vehicle. OR, Martin stopped and Zimmerman pursued him, then a confrontation ensued and either Martin or Zimmerman initiated physical force.
If Zimmerman's story is accurate, ALL fault lies with Martin. If Zimmerman caught up to Martin, then initiated force against Martin, likely Zimmerman should go to jail and the fault is ALL on him. If Zimmerman caught up to Martin, then Martin initiated force, then Zimmerman should still walk, but there would be fault on both sides.
However, as far as we know there are zero witnesses or evidence that disproves Zimmerman's story. In absence of all evidence to the contrary, the only real option is to not even charge him.
Perhaps there is another element in this that we haven't considered.
SPECULATION ALERT!!!!
Z., from all accounts, is a fairly liberal/progressive-minded sort. We've also been told that he had initiated telephone contact with the Special Prosecutor without the consent of his lawyers (who have since excused themselves from his case). Z. may have decided he was at fault here and may have given the Special Prosecutor a statement that justifies his arrest and indictment. Such a statement may not have any particular relation to the actual facts of the occurrence, but may be motivated by his guilt at taking a life.