While prosecutors argue that Zimmerman's statements to the 911 operator about the "f------ punks" always "getting away," shows ill will, most legal analysts felt from the beginning that with a fight, a murder charge was overreaching.
Manslaughter is far more likely to create debate in that jury room (there could also be even lesser crimes they consider, where they could find him guilty of something).
Zimmerman's injuries alone -- his broken nose and cuts on the back of his head -- are objective evidence to support his account that he shot Martin as he was being pummeled.
Just as important is the testimony of neighbor John Good, who lived directly in front of the location where Martin was shot. He very precisely (but reluctantly) testified that he saw the lighter skinned man in the red jacket on the bottom of the scuffle with the darker skinned man with the darker clothing on the top in a "mixed martial arts position." He said he now believes that Trayvon Martin was on top of Zimmerman.
But wait, another witness said she thought Zimmerman was likely on top. Put aside the fact that Good's home is the closest to the incident and that her testimony didn't seem nearly as credible or definitive as Good's, that doesn't change the legal reality that one does not negate the other.
The prosecution has the burden to prove the case and so if there is reasonable doubt, the defense wins. Good's testimony in conjunction with Zimmerman's injuries are likely enough to cast reasonable doubt on the key question, which is whether Zimmerman reasonably believed he needed to shoot Martin to prevent "great bodily injury."
Of course, the jurors could also accept all or the vast majority of Zimmerman's account, making an acquittal that much easier.
What about the fact that prosecution witnesses have testified that his injuries were not that significant? While interesting (and debatable), the only relevant legal question is what was Zimmerman thinking or fearing at the time, not what already occurred.
In many self defense cases the person who shoots a fatal bullet suffers no injuries at all and instead argues he or she protected himself or herself from injury by shooting the attacker.
So wait, let's take a step back. If jurors believe Zimmerman followed Martin, maybe even racially profiled him and initiated the altercation, can Zimmerman still legally claim he needed to defend himself and walk free? Yes.
If these jurors have questions or doubts about whether, at the moment he fired the fatal shot, Zimmerman "reasonably" feared that this was the only way to stop from getting beaten further, then they have to find him not guilty.
To be clear, if we were talking about Florida's controversial Stand Your Ground Law, who initiated the encounter would be crucial and the defendant would have the burden to prove that he should not be held legally responsible for the shooting. That law, which can protect a shooter from even going to trial, wasn't designed for someone who starts a fight and then loses the fight he initiated.
Zimmerman waived a pre-trial Stand Your Ground hearing and went directly to trial (likely because his lawyers knew they would lose) and simply argued classic self-defense, which is different. Now no matter how it started, if Zimmerman shot Martin because he reasonably believed it was the only way to protect himself from "great bodily harm" then he is not guilty. That's the law.
With all of this said, juries are notoriously impossible to predict and the deliberation process can take on a life of its own, but if they follow the letter of the law, it's hard to see, based on everything we know now, how they find him guilty of either murder or even manslaughter.