To be clear, the topic of this thread is stupid. So...
I guess it's been so long, that the last time I went on a tangent about word origin was on Lithium. There will always be certain words that can not be anything other than what they are. The word n****r was created to be dehumanizing to blacks, not just an existing word adapted to be dehumanizing (like f****t). In this case, it's a bastardization of "negro," being transformed in the process of being converted to English based on the sound of the word rather than the meaning. I know this isn't exactly you point, but it's worth mentioning that the word can never be used is because its impossible to redefine a word to completely eliminate its original usage. Language adapts by building on to a word's definition - a etymological chain is created where the word's definition is expanded by a change in context that has some logical connection to the base definition.