So a ruling from the Judicial Branch (ie, the courts) results in more perceived injustice than no court case, and is thus more justified? I guess civilization hasn't come as far as we like to think it has.
As for the question, the reason people think (members of) the black community will riot is because a) people remember Rodney King, b) the media turned the case into a hunt for blood, and c) there have been tons of posts on twitter and other social media calling for blood/riots if he goes free. Sure, I don't expect every one of those posts to be sincere, but their existence does lend credence to the likelihood of riots if Zimmerman is found not guilty.