Not nearly enough reliable public evidence to establish guilt, especially for second degree murder. In our system, innocent until proven guilty. AFAIK, there are four eye witnesses, two during the "scuffle" and two immediately after. There are several ear witnesses.
In court, the main eye witness is going to carry more weight. That person has Zimmerman underneath Martin. He claims the one in the red sweater (Zimmerman) was looking at him and yelling for help. Then he claims when he returned from calling 911, the one on top was now face down in the grass, apparently dead. Also, the initial police reports indicate Zimmerman with a bloody nose and grass stains on his back.
The second real-time eye witness was further away apparently. This person says it was so dark that he/she couldn't actually see much movement but that it appeared there might be a scuffle, and that it looked like the larger man was on top. Although the media is making it seem like there was a big size difference, in reality Martin was 6'3" 150-160lbs depending on the source and Zimmerman is 5'9" and 170lbs. Who is "bigger" in this case is subjective. Especially if it was two dark to actually see what was going on from that distance. A distance that is also confirmed by the fact that he/she said the gun-shot was more of a "pop". Given that the shot was very loud and audible on the 911 call near the shooting, it would suggest that this eye witness was a pretty good distance away.
The two other eye witnesses showed up immediately after the gun shot. They saw Zimmerman standing over Martin and then pacing with his hand on his head. Martin was face down, and Zimmerman made no attempt to turn him over, according to them.
The ear witnesses all confirmed that one of them was yelling for help, but they cannot reliably identify which. Martin's mother insists that it is her son's voice. Martin's father originally told police it is not his son's voice, but later retracted that statement. Voice analysis is trying to match the screams with Zimmerman's previous calm-voice 911 call, but say there is only a 45% match. But again, the main eye witness identifies it as Zimmerman's voice, because he could hear and see him.
According to the 911 call, initially Martin was staring at Zimmerman and started approaching him "to check him out, with his hand in his waistband".
As much as people want to insist that Zimmerman ignored the 911 dispatcher and kept following Martin, that's all supposition if you listen to the tape. The dispatcher suggests that Zimmerman doesn't need to follow him, and it sounds very much like he stopped moving. The wind in the phone stops completely. He also makes a statement in the past tense.. "He ran!" Although he could have resumed things after hanging up, there is no evidence of that at this time.
From this evidence, a 2nd degree murder conviction would be a tough sell. IMO
The comments/questions that I have are...
1) Although it sounds like "Z" stopped his initial pursuit, I would assume that he probably at least casually continued looking around. Especially since he would not tell the dispatchers where he would be waiting, but instead said to have them call him, as if he intended to keep looking around. Again, this is supposition, and it also is not a crime to go looking around for what might be suspicious activity in your neighborhood. But if he was returning to his car, he must not have been very close to it yet because the fight and shooting happened on the lawn behind the buildings.
2) "Z" must've suspected that "M" might still be nearby because when asked his address he only gave 1/2 of it and then stopped. He then expressed his concern that "M" might hear where he lived if he continued.
2) If "M" was on bottom during the fight and was shot in the chest, why do witnesses and police put him face down. He should have been on his back. Witnesses verify that "Z" made no attempt to turn him over. This very much suggests that "M" was on top during the fight.
3) The two after-the-fact eye witnesses say "Z" was pacing with his hand on his head because he was in shock. That is pure speculation. Just as likely, if not more likely, seem to be that he was nursing injuries. After all, police reports confirm the head injuries and grass stains on his back. What do you do when you hit your head or smash your nose. You cover or rub the wound.
4) If the above is true, where is the blood that would probably be on "Z" after the shot? It is hard to think that there wouldn't be blood on him, but it is certainly possible. If say "M" was not directly over him. We sort of assume that he was directly on top of "Z" when the gun was fired, but there really isn't any statement that I'm aware of to support this. Also, we don't have any record stating that there wasn't blood on him either.
5) The voice yelling on the 911 call sounds more like a young man's voice. Unfortunately we have no record of Martin's voice to compare.
IMO, the hard evidence supports most of Zimmerman's account. The softer evidence (like martin's mom identifying the voice as her son on the 911 call) supports Martin. If this is really all the evidence we have, then Zimmerman cannot be convicted. But I'll agree that a couple things don't add up.