OJ probably killed Nicole and her boyfriend, but got off because the jury somehow found reasonable doubt as to whether it was OJ or somebody else.
Martin assaulted Zimmerman and likely intended to finish him off by pounding his Zimmerman's head into the concrete sidewalk. The jury had a ridiculously easy case of aquitting Zimmerman because the evidence was overwhelming that it was self defense. The only reason there Martin is not seen as a street thug who got himself killed by an innocent watchman is because of a bigoted hate campaign.
Neither of these cases ought be about race, and I don't think it a good thing to counter one with the other. The answer to those who are bigoted toward whites is not to put down blacks.