You can say whatever you want, to whomever you want. It's probably not a good idea to say "nigger" or "nigga" to or about a black person, if ever, if you're a white guy, especially if you're not keen on offending, hurting, condescending to, or putting down someone.
The word has evolved the way it has. That's just where we are. There's myriad opinions on the word, in any number of ethnic communities.
Why does it upset you so much that other groups "can" say what you, a white male, "cannot" say?