I would be careful about using the GF's statements about what she heard -- she has Zimmerman still following Martin which kind of kills the "he was going back to his truck" stuff.
I can think of a number of reasons why Martin might have "beat on" Zimmerman, but the one that springs to mind first is that Zimmerman was a strange adult male who got out of his car to follow him. If Zimmerman showed or reached for the gun then Martin would have been fighting the strange adult man with a gun who was following him.
Maybe that happened and maybe it didn't. The thing is that I'm not swearing that it did, whereas you and a bunch of others are swearing that Martin jumped Zimmerman without provocation. I say prove it.
Actually, that's a bit backwards. Zimmerman is charged with second degree murder. According to the legal analysis I have read and the Florida laws, the prosecution is expected to prove beyond a reasonable doubt that George Zimmerman acted with a "depraved mind" when he shot Trayvon Martin, that Zimmerman acted with ill will, hatred or spite. With only one living witness to the start of the fight, that is almost certainly impossible. This story should die or be used as an example to discourage criminal behavior, but a trial is absurd.