But the jury isn't being asked to decide whether an "assault with a deadly weapon" has occurred. They're being asked to decide whether or not the killing of Trayvon Martin was an act of self defense (and thus unfortunate, but legal under Florida law), or an act of 2nd degree murder (and thus illegal, and carrying with it a stiff prison sentence).
An act of self defense is an affirmative defense. You say to the jury, "Yes, I shot this person. But this why..." Self defense is legal in all the states of the Union. Why would it be unfortunate? People seem to think that Florida is somehow special in that you can shoot with impunity even if you just "feel" the least bit threatened. NOT TRUE.
Take New York State, for example. NYS (where I live) has no stand your ground law. The relevant statute says you have a duty to retreat when in public. BUT, it doesn't apply if you can't do so safely, e.g., I have a toddler with me, doing so would mean running into an active highway, I'm in a wheelchair, I'm wearing a cast on an appendage, etc.
In Florida, you have no duty to retreat if you are in a public place where you have every right to be. But all the other other legal standards for acting in self defense still apply. That is, was I the initial aggressor in the confrontation? Did the attacker have the ability to attack? Did he have the opportunity? Was he placing me in jeopardy? Read here: http://www.useofforce.us/3aojp/
Since all we have is one side of the story, past patterns of behavior on the part of Martin & Zimmerman may be very relevant in assessing the evidence. *IF* Martin has a history of breaking into houses, getting into fights, etc. etc., then it makes Zimmerman's story - that he was standing there when Martin approached him and assaulted him - somewhat more believable. If Martin is shown to be the poster boy for good kids everywhere, then it makes it far less believable. Just as past patterns of behavior on Zimmerman's part are relevant - does he have a history of racism? does he have a history of assault? does he have a history of waving his gun around like a maniac? All of these things would make his story LESS believable.
It's all relevant, because there simply aren't many facts beyond "deceased young black male, shot at close range" and the defendant's claim that "I was jumped, and acted in self defense." What a jury is being asked to decide is - is Zimmerman's story reasonable?
Not only that, but the legal standard would be: what would a reasonable person do at that time under those conditions knowing what the accused knew?