My father once called a man "kike" in a shouting match because in his rage he reached for any word available to use as a weapon. The word didn't really have any antisemitic content in the mouth of my father, a proud Jew, but it was a blunt instrument that HANDED ITSELF to my father, and he used it.
Something like this seems to have happened to Michael Richards. The language of racial hatred spoke more through him than from him. At least, I think that's probable. I don't think this was the case with Mel Gibson a few weeks ago, though. Gibson's past public comments, his religious beliefs, and his films all support the reasonable conclusion that the man is grossly antisemitic. Michael Richards may turn out to be genuinely racist, too, but I doubt it.
The language running through our heads is sometimes our own, sometimes the zeitgeist's, and telling the authors apart can be tricky, especially for an improvisational performer. Richards went momentarily around the bend, that's for sure. What this says about him isn't so certain.