In general 1st degree murders are premeditated ones, i.e. the person planned days in advance. It doen't apply at all here.
I thought it they would go for manslaughter, but they chose a tougher one. Most likely they will try prove that it was a fight with both side participating in it. Wild guess - they would make an analogy of a bar fight, both sides are guilty of fighting. Since Zimmerman killed his opponent, it's a 2nd degree murder.
Remember if some a retard on a street comes to me and starts asking me questions, I am legally within my rights to tell him go fuck himself. Zimmerman, I assume, got pissed about that and got in scuffle, by trying to hold Martin which resulted in punches traded and Zimmerman shooting Martin.
^^^ Just a bar fight, with one idiot having a gun.
Prosecutors just need to prove that Zimmerman challenged Martin (verbally), it doesn't really matter beyond that point.
Doesn't the "stand your ground" law mean the exact opposite of that? If Zimmerman felt threatened, wasn't he entitled to kill?