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The standard of proof I "demand" is called just that - proof.
You have just acknowledged that you don't have it.
I could take the exact set of knowns (zimmerman followed martin - martin ran) and say that the fact is that zimmerman confronted martin, showed or reached for his gun and martin started fighting for his life by knocking zimmerman down before he could pull the gun and continued to beat the armed stranger to save his own life.
That version is based on known facts --- therefore it must be true.
But there is additional evidence, including: Zimmerman's past conduct toward people he called the police on; Zimmerman's physical shape as evidenced by getting winded after a run of under half a minute; the relative physical condition of Martin compared with Zimmerman, in particular speed and endurance in running, reach advantage, etc.; Zimmerman having called the police and was expecting them; absence of any witness hearing "Don't shoot!", "Gun!", or similar; that if Martin did see a gun before he hit Zimmerman (as your hypothetical posits), he failed to focus his attention on the gun arm for the duration of the one minute or longer altercation.
-- The standard of proof I "demand" is called just that - proof. --
I understand. You require a reliable, credible, and independent witness, or video. A person who was on scene, who viewed the entire altercation from a vantage point that admits seeing and hearing the action, combined with an ability to clearly recall the sequence of events. But the police and court don't have that, so have to reach a conclusion on the evidence they do have. The police didn't have a problem doing that. The state, in order to mount its case, has introduced two witnesses that the SPD did not use - DeeDee and Sybrina. The state's theory of the case depends on the testimony of those two, which runs counter to the testimony of on-scene eyewitnesses.
-- You have just acknowledged that you don't have it. --
Nobody has it, or will have it. That condition of quality of evidence is no big deal, despite your attempt to make it so. There is plenty of evidence in this case. That the evidence does not meet your standard for finding Zimmerman to have justification for his use of deadly force is irrelevant, because the law provides its own standards of "proof." At a 776.032 immunity hearing, the standard is "more likely than not," and if the case goes to trial, the burden is on the state to disprove the use of self defense, beyond a reasonable doubt. Neither of those standards is "proof to a metaphysical certainty."