Person X is in an argument with an African American man, and the African American is frustrating Person X to the point that Person X feels very angry and dislikes this man (solely based on the argument and nothing to do with colour). Person X wants to hurt this man because he hates him, so he says the most hurtful thing he can to disparage this individual, so he makes a racial slur. Let's just say he uses the word nigger.
Person X is using a racial slur to hurt this man, but does that make him racist?
In my opinion it does not. There is a big difference between saying something horrible in the heat of the moment (Michael Richards) and having the belief that one colour or creed is superior/inferior to the rest (Hitler).
Person X certainly used a racial slur as a racist comment, but was it racism?