I think there's a lot of misinformation going around. The most impartial explanation I've seen that covered all of the facts came from reddit, and I pasted it here:
Zimmerman claimed that while he was walking on the property, Martin jumped him, knocked him down, was beating him up, and tried to take his gun, at which point Zimmerman shot him.
The witness with the best view of the fight saw one person on top of another, beating him. He believed that the person on the bottom was wearing a red shirt. Zimmerman was wearing a red shirt (or jacket or sweatshirt).
Zimmerman had cuts on the back of his head that are consistent with it being hit against a sidewalk while he was on his back.
The coroner testified that the gunshot wound was consistent with Martin being shot while bending over someone.
Now for my opinion:
If you ask me, Zimmerman chose to kill Martin. I don't know whether it was based on race, but based on the evidence there was not a good reason to use deadly force.
Unfortunately, based on Florida's law, this is perfectly legal.
So it looks like Zimmerman is legitimately not guilty based on the letter of the law. He may not be guilty, but he's from innocent...