Where you’re getting cobbled up is by assuming that somehow a homicide victim will have “this was justifiable” tattooed to his forehead, and that nobody would ever falsely claim the homicide was justifiable. So you’re starting out having already assumed the homicide was justifiable, whereas nobody else involved knows whether it was or not.
Again, when they have probable cause to believe that one person violated the statute prohibiting the UNLAWFUL killing of another a charge is filed. Accident or justification go to the issue of whether the killing was unlawful or not. If the prosecutor does not believe the death was accidental or justifiable, a charge is filed.
In other words, there’s no such thing as a charge that says, “we believe this guy lawfully killed someone”. In that case, no charge would be filed.
All anybody can go on is evidence. Or at least that’s how it’s supposed to be.
Can you think of evidence that would undo the evidence of the witnesses, 911 call, and Zimmerman’s injuries? Obviously if there were irreconcilable discrepancies in the accounts or evidence that would negate their value. But surely the police would have been looking for discrepancies as well.