The bar for a federal case is set very high in terms of evidence... A DoJ prosecutor would have to to "prove" that Zimmerman wanted to kill Martin because Martin was black... This idea of "proof" when there are no witnesses to the crime is all but impossible unless Zimmerman had told someone that he was going out to "kill a nigger"... That's the level of evidence that would be required for the feds to take the case...
They took the Rodney King case because of the video... Had there not been a video then there would not have been a case, even if a witness had come forward because the white cops, like Zimmerman has done, would have created their own story and stuck by it...
The problem here is that these "stand-your-ground" laws are too open ended... The concept of self-defense has been around forever but "stand-your-ground" is relatively new and pushed by the NRA to sell more guns and make people fearful...
I mean, if I know that someone wants me dead and that all they have to do is make sure there are no witnesses when they shoot me and then claim they were scared that I might harm them, regardless of the facts, then, yeah, makes me want to carry a gun just to shoot back when some asshole thinks he has the right to shoot me 'cuase he doesn't like me...
These laws must go... They may sell a lot of guns but they are making everyone a target for someone who wants to kill them and knows they can get away with it...