Let me preface this by saying that I'm not as well versed in Florida law or even the particulars of the trial as many here. My perspective on this is that of a layman:
It seems to me that only two people know for sure exactly what happened between Zimmerman and Martin, and one of those two people is dead. Given Zimmerman's current situation, we obviously cannot take his story at face value due to his obvious motivation to lie if he is indeed guilty of what he's been accused of. Because of this, I tend to give the most credence to the physical evidence as I believe that is probably the most unbiased kind.
The nature of both Zimmerman and Martin's injuries suggest to me that it was Martin who was the aggressor in the physical altercation rather than Zimmerman. Had Zimmerman started a physical altercation with Martin, I would think that Martin would've had more injuries than abraded knuckles and a single fatal gunshot wound. Therefore, it seems to me that the physical evidence corroborates Zimmerman's side of the story.
Now, it is certainly true that Zimmerman may have initiated the altercation with Martin in a non-physical way and that Martin escalated that altercation to a physical one. That being said, I would think that Zimmerman would have had to confronted Martin in such a way that Martin had imminent fear for his own physical safety in order for escalation to the physical level to be legally permissible. Simply demanding to know who Martin was, where he was going, or what he was doing does not seem to me that it would justify physically attacking Zimmerman. About the only non-physical method of confrontation that I can think of which would justify Martin attacking Zimmerman would be a threat of physical harm, and there's no evidence that I've seen that Zimmerman made any such threats.
Now, please don't misunderstand me. I certainly feel that Zimmerman made several poor decisions hence his current situation. However, I've seen no compelling evidence that Zimmerman's actions, while obviously unwise, were in fact criminal.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar