While 'nigger' may not be a bad word, it isn't exactly complementary. In fact, it's downright insulting. It's a racist word. Using that word is no different than using honky or cracker.
It's also a word that will get a reaction quicker than most. Really, the word goes against the site rules of vulgar, hateful, obscene, profane, defamatory, and abusive. Not really a word we want here.