It's complicated, but not really.
Self defense is an affirmative defense; you're saying that you did something that would ordinarily be wrong -- but you had a good reason or good justification. The Zimmerman case, based on a lot of coverage is not the situation you've described. Manslaughter is, I think, a more viable charge here than murder. Manslaughter is death by misadventure; the defendant gets into a situation where they could and should have recognized the danger, and done something else, but they ended up killing someone. Murder requires malice aforethought -- an evil intent to do harm, even it if only formed a few instants before the attack. That's where most drunken driving killings aren't murder; the defendant can't form the required intent -- but they could and should have known that they shouldn't have been driving. Honestly -- it doesn't even seem that Zimmerman initiated the contact or started the fight.
So... how could an aggressor become a victim, and be justified in using force, especially lethal force? Basically, the original defender has to become an attacker. One simple way, that happens too damn often, is the homeowner who defends his house or the cashier at a stop & rob who defends himself against a robbery -- but then goes in chase when the bad guy runs away after being thwarted. Suddenly, the bad guy is fleeing from a guy with a gun -- who has now become the attacker, and could easily find himself in a jam. Another slight variation that's just especially relevant to martial arts instructors is a student who defends himself from an attack. The defense is successful, and the attacker is down, helpless, and the student throws in another shot, or just carries that wonderful self defense combination through to the head stomp, throat rip... and finds himself arrested.
