I don't think I agree with that. In practice self-defense is really only a quasi or nominal affirmative defense in that a defendant only has to raise it and then the burden shifts to the prosecution to disprove it. Certainly that's how it is in my state and IIRC pretty much every other one I'm familiar with. Are you sure Florida treats it as a full fledged affirmative defense?
That's just plain weird. If the burden is on the state to disprove self-defense, beyond a reasonable doubt, like I think it is, then that's a harder burden on defendant -- but it sure as hell seems like a chance you'd want to take. If the judge really is allowed to weigh evidence and determine credibility, then all he has to do is decide Zimmerman's account better than the proof against him, and he'd have to let him off. Considering the only real proof the State has is suspect (the combination of the belated account of the not-credible girlfriend and the mother's not-credible claim that it's Martin, rather than Zimmerman, screaming for help on the tapes). Given that Martin obviously wasn't running away and didn't have a scratch on him other than a bullet hole and bruised knuckles, Z should get off. I feel like maybe we're missing something here.
You're missing the political realities of the case. The judge could not possibly dismiss the charges under SYG and hope to not be heckled/recalled/impeached/whatever they do in FLA. Why show your case to the Prosecution ahead of time when you are confident the judge will disregard your presentation anyway?
Also the judge in this case is a female.