I'd be inclined to agree with you here...but, the dispatcher explicitly told him not to engage, that authorities were on the way...yet Zimmerman still engaged. With a handgun on his person.
Self-defense is a very, very tough stretch for me. In my opinion, Zimmerman gave up his right to hide behind the 'stand your ground' bullshit the moment he exited his car.
I completely agree he should not have engaged him, but again, to what extent was it? Also, and this is purely curiosity, but is there any laws against disobeying a dispatcher's orders?
Self-Defense will be based entirely on intent and motive of Zimmerman leaving his vehicle in my opinion. It is a very tough matter to decide and I personally am not a fan of the "Right-to-Shoot" laws. I think what can be said is Zimmerman is completely morally responsible for the death of Trayvon, but depending on when all the facts are present, it's a tough call rather he's legally responsible if the witness' report is accurate.