Most likely because one of them is black. If both were black, or both were white, no one would be making a big deal-but because a white guy shot a black guy in self defense it's instantly racist and the guys "head should roll". If it was a black man who shot a white man they'd all be saying how zimmerman was attacking the guy and he had pure rights to defend himself. It's almost reverse racism. Which is actually a thing. Race should have absolutely ZERO place in politics and law.