Well from what I've looked up, Zimmerman has the burden of proof to show that Martin actually attacked him first, which is going to be an uphill battle given his aggravated speech and aggressive pursuit of Martin while Martin was attempting to flee. Furthermore he apparently had the gun in a visible holster while pursuing Martin, which would actually mean that Martin was justified even if Martin attacked him first.
That being said, I have been doing my amateur google-legal research, and in regards to sentencing, it should be noted that I've found that in Florida, the crime will apparently be elevated to the next degree
because a firearm was involved, and on top of that it
involves a child and actual aggressive behavior by the killer, in which case I think we're starting with a baseline of voluntary manslaughter that can end-up as second-degree murder. Which is life in prison.
Bye-bye Georgie. It's not looking good.