It's very nice of you to act like a lawyer, but none of this matters. The only thing in the world in court that will excuse physical violence is proof of self defense. In this case, it wasn't self defense, and there's no proof of a racial slur. Whether you like it or not, the country still protect speech that isn't a threat. While it's wrong to insult someone, calling them a name doesn't equate to self defense. There's a reason the KKK can publicly hold rallies and have no crimes against them. They are very careful to not make threats and not participate in physical violence.
In the end, there's a piece of evidence that someone said "piece of crap" and there's nothing else. Furthermore, if you actually watch the video, you can see Smart turn around less than a second after you hear "You piece of crap!" It doesn't *prove* anything, but it's evidence that goes against Smart's word.
Make an argument by defending the KKK - that really gives you a lot of credibility.