I don't question his right to defend himself either, but chasing someone around in a car and then getting out to confront them isn't 'defending' yourself. The legislator that created the 'stand your ground' law in Florida was on the radio this morning, and he said the law isn't intended to protect someone who pursues someone else and initiates a confrontation.
Florida's Republican legislature isn't about to remove this law from the books, but Zimmerman's actions are going to make it a lot harder for people in other states to get a 'stand your ground' law on their books.
And that is the left's purpose in making such a big stink about this event. They don't like that people should have the ability to defend themselves against thugs and such, so they've taken a case where someone (allegedly) went beyond the law and twist it into an attack on the law itself.
Thugs hate armed citizens.