I'm starting to wonder if George Zimmerman isn't racist (or racism didn't play as much as a role as we thought) and just has a hero complex.
I read
this article on Cracked yesterday and feel that it might be relevant.
#2 "People Who Crave Attention Will Make Bigger Sacrifices" discusses a study where altruistic behavior was linked to a desire for notoriety rather than altruism. I don't know how valid it is (it's a Cracked article, after all) but if it's true it disputes the idea that Zimmerman is automatically an upstanding man who couldn't have murdered Martin because of his actions today.
With a few exceptions, people who do evil don't consider themselves evil. Zimmerman thought he was doing the right thing when he followed Martin.