I'm thinking this whole thing got twisted out of content and truth because Zimmerman used a racial slur when he called the Non-emergency number to report Martin as a suspicious person and he was following him.
Zimmerman was told not to follow by the dispatcher.
From what we can gleen from the information we are given is...the 911 call with the "screams" on it was from the witness and not from Zimmerman or Martin.
It seems Martin may well have jumped Zimmerman from behind s he was headed back to his car.
Zimmerman does have injuries and other forensic evidence to show he was at least in some sort of fight.
Now the key evidence that will show if this is self defense, murder or manslaughter will be the ballistic results.
If Martin has little or no gun powder residue on him that would suggest he was shot from a few feet away.
If there is a lot of residue and the pattern is is tight that would show he was shot right up close and more or less back up Zimmerman's claim.