Yeah, I don't think Zimmerman was going out looking for a fight. All the pundits said today that the prosecution was trying to get in his 911 calls throughout the previous 6 or so months to develop a pattern of civility and now all of a sudden he's a vigilante. If I were the defense, I'd gladly accept those calls as evidence that the police didn't do their job, so on this night, Zim felt like he had to follow the kid all the way till the end until the cops showed up. Martin turns around and instigates a fight, the gun goes off.
It's a story of lazy police work to arrest someone who had broken into neighboring houses, frustrated citizens, mistaken identity (Martin didn't break into houses in my opinion), and an unfortunate accident.