For the defense's argument to work, they don't have to show Zimmerman injured Martin (to whatever degree you would require), they just need to show that 1) it is reasonable to fear for your life in Zimmerman's situation and 2) that Zimmerman had no avenue for escape when the shooting occurred.
I haven't studied legal precedent in Florida enough to know one way or the other, but I strongly suspect (am almost certain) that it's reasonable to someone to fear GBH and/or death while being straddled and struck. Also since Zimmerman was on his back with Martin on top of him, there was no avenue for retreat. That matters because Stand your ground hasn't been used by the defense in this case (despite what many would have you believe).
To sum up: In the eyes of Florida law being straddled on the ground and struck repeatedly, creates a situation wherein a reasonable man would fear for his life (or GBH). Martin being on top creates the legal equivalent of the threat posed by the knife in the hypothetical I mentioned before.
He didn't shoot before Martin got on top of him because he had no reason to fear for his life at that time time. Martin getting on top and continuing to strike him presented that threat which he then responded to.
That is the one act that Zimmerman was smart about.
I find it interesting that you aren't getting this. This is day one law school stuff.
You can make presumptions about my agenda all you want in an effort to impugn my character, but the fact is that I care for the law. Especially considering that it is as deaf as an adder to the clamors of the populace.