I don't think it's reasonable to believe that Zimmerman's life was in danger. I also don't think Zimmerman did anything at all to try to keep Martin alive after he shot him, which is pretty awful, considering that Zimmerman wasn't even in danger of getting punched a few times at that point, and he was the party responsible for the death.
The question is whether Zimmerman, at the time he was getting beat up, reasonably believed that he might be killed or seriously injured if he didn't put a stop to it. Awful tough to say no, beyond a reasonable doubt, without being in that situation.
I don't have a problem with GZ not rendering first aid to the guy who just attacked him - GZ would probably not have knows how seriously injured TM was by the one gun shot.
The one part I have a problem with is GZ saying that he got on top of TM to restrain him after shooting him. If I was in that situation, I probably would have gotten the eff away from TM as soon as I was able. But not having been in that situation, I could be wrong. That's the problem - you just can't know until you're in that situation.
It's just like TM purportedly saying "you got me" after being shot. Is that really so hard to believe? How the eff do you know? I'd like to think I'd go all Rambo after getting shot, but more likely, I'd probably just be in shock and say something dumb like "you shot me."