Riley Cooper screwed up, plain and simple. He used a derogatory term, meant to dehumanize black human beings. The incident of him using the slur at a concert last month was captured on camera, and exposed this week.
Cooper is not the only professional athlete to get caught using unacceptable, dehumanizing language directed towards some subset of the population in the last several years.
In 2003, Lakers' center Shaquille O'Neal mocked Yao Ming's Chinese heritage, using language representative of ignorant stereotypes of Asian languages.
In 2011, Shaq's former teammate Kobe Bryant used a homophobic slur, directed towards a ref.