Hmm, sensitive topic. I think it's hard though to label someone, who you've never met, as racist just because of the use of one word. I'm white and have a black friend. Me and my friends call him 'n***' sometimes, he doesn't mind at all, we joke around all the time.
It's all about context. I'm never gonna call a black man I pass by on the street a 'n***'. But between friends I sometimes use the word, and won't stop doing it because someone thinks it's racist(I'm kind of stubborn). I do it always jokingly, never to offend and I'm definitely not racist. (Apart from the subconscious prejudices I'm sure we all have )