I think that for M. Richards to say that he is "not a racist" is ridiculous! Maybe he doesn't think he is, or pretends not to be but deep down, those words and feelings had to come from somewhere. Granted, it wasn't right for the hecklers to do what they did either, but surely Richards has been heckled before - what comedian hasn't, at some stage of his/her career? You have to have some standard lines to use in a case like that.
I think his ego is very bruised from not having been successful after "Seinfeld", and those hecklers just touched a nerve with him...but it is unforgiveable.