Now, how to explain these reactions?
The fact that Zimmerman stinks so badly at his self-appointed job of "neighborhood watch captain," it led him to kill an innocent man. (If you think Martin was actually committing a crime, I can probably give you 2-to-1 odds.) In the absence of social condemnation, Zimmerman might continue carrying a gun in pursuit of his hobby. The media has put itself in charge of condemnation. Since some of their leaders seem very stupid, we would expect them to make mistakes. (In fact, you could argue they stink at this job and shouldn't be in charge of condemnation at all.)
The fact that Florida law might allow two men to "stand their ground" and escalate the use of force until someone dies also seems relevant. If it actually happened then Florida voters should know. But I don't think this played more than a small role in the media's actions, because they seem too stupid on the whole to make decisions based on policy concerns.
You are aware that according to the best theory based on public available evidence Zimmerman didn't shoot Martin until the latter was beating his head on the pavement? Also in that kind of situation "stand your ground" laws aren't all that relevant since there is no way to retreat.
Edit: Also, your "media incompetence" theory doesn't explain why all the mistakes went in the same direction.
I have found this most wonderful (if fairly lengthy) article, and thought you would enjoy having it brought to your attention.
It is so good, I think we should include it among the references in the "Politics is the Mind Killer" wiki page. But, before that, I submit it to you, and ask you: is there anything in this article that would warrant its exclusion from this site? I mean, besides the fact that it is about politics, and written by a notorious Social-democrat (And is that in itself grounds for exclusion?).