JR is the better one to ask, but...
Generally, unless you are in your home, if you have a gun and they don't, and they don't make you believe they have a gun, then you are in the wrong. You can't use deadly force unless in a neutral area (at all in some States) unless the same is being used against you. Fist a cuff's wouldn't be an exception...Unless they were Mike Tyson or some shit.
Regardless of all of that, he was following the kid. Vigilante-esque, even after the cops told him not too. He instigated the majority of it.
Also, I agree it should be manslaughter. I don't know why it is a national story either. Well, I mean it is retarded that the DA hasn't yet charged him. Even if the charges aren't successful, they can say they tried. The guy is a dumbass who had an itchy trigger finger and should not have shot the kid.
FL recently passed a "no retreat" law. Used to be, outside your home, you had a duty to retreat from attack, if you could. Now you can stand your ground.
The facts in this case are, for now, what Zimmerman says, and witnesses. I understand there's one witness that verifies Zimmerman was on the ground being kicked.
To use deadly force, you only have to have a reasonable belief that your assailant is about do cause you serious physical injury or death. IMBLO, being on the ground kicked in the head by a 6 ft tall 17 year old, could easly qualify.
Zimmerman did nothing wrong in following a suspicious character in his own neighborhood, and it troubles me to hear an American citizen claim "that's the job of the police". No, we're all responsible for our own safety, and I like it that we have people that take community safety on as a responsibility. Zimmerman had, in the past, made some 200 calls on suspicious activity in the neighborhood, and never had any kind of trouble due to it.
There's a minute of time, that only Zimmerman and Trayvon know what happened, and Trayvon is dead. I've heard the unredacted 911 call (haven't read this whole thread, it may be in here). Zimmerman sounded calm, and didn't make any threats, and appears to have lost sight of him just before he hung up. If Trayvon circled back around and confronted Zimmerman, then I think Zimmerman probably gets a pass
based on the currently known facts.. There will still be forensics, and they could tell a different story, or tell if Zimmerman's account doesn't match forensics.
I'm reserving final judgement on forensics, but as of now, with the known facts, I'm not saying Zimmerman was "right" but he probably will be legally cleared.