All we know from the article is that Martin was shot from very close range.. the two were on the ground and it’s plausible that they struggled for control of the gun.
I’m sure Martin didn’t notice Zimmerman was armed until he was underneath him. Which certainly doesn’t make Zimmerman trigger happy, as he would have pulled the weapon much earlier. It’s highly likely Zimmerman felt at risk of himself being shot by the weapon.
One could argue that if he hadn’t been carrying it wouldn’t have come to this... but I don’t see where the police had any choice other than to release him.
That's right. Zimmerman would have been just another anonymous murder statistic, instead of the target of a Black Supremicist lynch mob.