Why didn't GZ do what you say you'd do? Was he looking for the ending that he got?
Not being George Zimmerman I can't tell you why he did what he did. I can try to make an educated guess based on what I know about Zimmerman's personality and the facts I do know about the conditions that night.
First of all, I don't see George Zimmerman as an "aggressive" person. All of the testimony...by both Prosecution and Defense witnesses painted him as the exact opposite of that. I believe the terms used quite often were "soft" and "meek". When Martin appears suddenly out of the dark with his "You got a problem?" query...Zimmerman responds with "I don't have a problem with you." and then tries to get to his cell phone to call the police.
What Zimmerman SHOULD have done was yell loudly for Martin to keep back as he pulled his handgun...letting the stranger approaching him know that he was armed and considered him a threat if he came closer.
It's quite obvious that Zimmerman ISN'T well versed in street fighting. He let's someone close on him without pulling his weapon or assuming a defensive posture. If he hadn't been such a wuss he would have learned about what distance is safe and what is not in the ring at his gym. If you're not throwing a punch or kick then you shouldn't allow yourself to be within a distance where your opponent CAN hit you with a punch or kick. Zimmerman got sucker punched by someone he SAW coming at him. That's the sign of someone who's NOT a fighter.