I’ve been following this case and have been frustrated by the lackluster (to say the least) performance of the prosecution and I’ve been reading (most of) this thread, and here, FWIW, are my thoughts.
Imagine this: You are walking home from the store on a rainy night and notice that you are being followed by someone in a truck. You run and get close to home and think you’ve lost them. But maybe you’re not sure, so you backtrack a bit to see, because maybe you don’t want this follower to see where you live (or any other reason really - because walking around your own neighborhood is not against the law).
Suddenly you are confronted by this follower on foot. Maybe he already has his gun drawn, or maybe he just grabs you or grabs at you and draws the gun later. He doesn’t identify himself as law enforcement or neighborhood watch. What do you do?
You could run, and risk taking a bullet in the back. You could shout for help, and risk being shot as well. Or you could charge him and knock him down and try to shake loose the gun from his grasp. Once you have him pinned down, then you can shout for help without the risk of immediately being shot. You are struggling to hold his arms down (hence the up-and-down motion) and shouting for help when he gets free and shoots you in the heart.
Did he intend to kill you from the start? Doubtful. Did the punching or grappling make him angry enough to shoot to kill? Maybe. Or maybe he realized that he made a big mistake by following and confronting you and that he’s going to be in a lot of trouble if this isn’t made out to be a self-defense killing.
A few other thoughts: If you were on top of an assailant and wanted to kill them (or quiet them violently), would you use your hands to try to smother them, or would you just wrap your hands around their throat and squeeze as hard as you could? If you were the follower (now on your back) in either of these scenarios, your arms would be free to hit the person on top of you or try to push them off of you or do something with them.
If you were Zimmerman and found yourself in this position because you had stupidly followed and confronted an innocent teenager without ever revealing yourself to be neighborhood watch, and you were able to somehow draw your weapon, wouldn’t you aim it at the kid’s face and say something to the effect of "Get the f— off of me or I’ll kill you? Would you issue no warning of any kind before killing an unarmed teenager you may have scared and cornered into this situation to begin with?