Much evidence and facts are already out.
Zimmerman had a broken nose, and bloody injury to the back of his head.
The back of Zimmerman's shirt was wet and had grass or grass stains on it.
The eye and ear witness, John, saw Trayvon beating Zimmerman. (the attack happened right outside his window)
The eye and ear witness, John, heard Zimmerman (no question it was Zimmerman) crying out for help.
The police did not arrest Zimmerman, and returned the gun to Zimmerman.
The police handcuffed Zimmerman and put him in the squad car to take to the police station. Technically not an arrest as they didn’t say “You are under arrest” and read him his rights.
They took his gun at the seen and still have it for evidence.
I have seen nothing so far that indicates that Zimmerman did not act in self-defense. The police have come to the same conclusion so far.