Zimmerman Verdict - Doesn't Make Sense To Me

This won't come as a surprise to regular readers of this blog (or wouldn't, if such people existed) but I'm mystifed how George Zimmerman is a free man today. Now understand, I don't have cable so I was spared the gavel-to-gavel coverage on CNN (by the way, I read that at one point, during the Egyptian coup CNN not only refused to cut away from the trial, but they actually had an inset box on the screen featuring some other aspect of the trial) so I can't speak to all the evidence the jury was exposed to.  But check me on the narrative that I understand the defense put forward:

Zimmerman, during his neighborhood watch night shift, notices a suspicious-looking character in the streets. He reports the guy to the police, who ask him to remain where he is and not pursue. Zimmerman pursues anyway, eventually stopping the car after a block or two and confronting the boy, who assaults him and bangs his head on the sidewalk until Zimmerman is forced to shoot the kid  in self-defense.

My question is, how is what Trayvon Martin did in this scenario NOT self defense? He was walking in a neighborhood he had a right to be in, and a creepy guy in a car is following him for blocks, then gets out and pulls a gun on him. What would you do? You'd try to at least knock him unconcious so you could take his gun off of him. I don't know what other choice a guy in his position was supposed to have.

Even given benefit of the doubt, Zimmerman should have been charged with manslaughter.