Self-defense is not “unlawful killing”, so to have a prayer of proving 2nd degree murder they’d have to prove that Z wasn’t acting in self-defense. And they’d have to have something to refute the 911 call, the testimony of all the witnesses, and Zimmerman’s injuries.
I don’t know what evidence would be able to do that.
Which makes me think that all of this "justice for Trayvon" talk from the prosecution is cynical anti-riot insurance.
For all their talk about "murder", I am certain that they will offer the jury the option to convict for involuntary manslaughter, which is what they are hoping for all along.
Self defense is an affirmative defense. If the defendant doesn’t assert it, and the deceased wasn’t executed by the state, his death is presumed to be unlawful.