Then we are in agreement. The accused does not have to prove self defense. There, that was easy.
Well it is could go down like this:
The DA shows evidence that the location of Zimmerman's phone call to the police is a far distance away from the location of the body.
The DA shows evidence that the police told Zimmerman not to follow Martin.
The DA shows evidence that Martin was unarmed and that the gun was Zimmerman's.
The DA shows evidence that Martin's girlfriend last heard him asking "Why are you following me?"
The DA shows evidence that witnesses heard Martin screaming for help.
And Zimmerman's lawyer doesn't present any evidence at all.
What that shows is that Martin was the one being assaulted, Zimmerman who introduced the dangerous element into the situation, and Zimmerman who caused the situation even when he was advised by the police to stay away. That provides the guilty mind and guilty act for second degree murder.
A good defense lawyer would find holes in that by finding evidence that suggests a different turn of events. That is what I mean by proving self defense, a defense that makes no statements at all will not prevail on self defense.