You have an eyewitness, the victim. The victim Zimmerman told the police on the scene, detectives and his father exactly what happened. You have evidence that collaborates his story... broken nose, bloody head, and grass stains on his back. A 13 year witness stating the Trevor was on top of Zimmerman beating the crap out him, etc... The screams for help on the 911 tape.
You have absolutely no evidence to the contrary (it might come out, but hasn’t yet). You are the one that is trying to make Zimmerman out to be a liar with no proof at all.
Or if you think it has been proven just because he, the shooter says so, then I guess that McDonald's worker in MD really won the mega-millions --- after all, she said she did. And, I am not 'trying to prove zimmerman a liar' at all. I am making the point that without any neutral corrobaration nobody knows what happened - not you, not al sharpton - nobody.