As far as I know, the n-word was deliberately invented solely to be used in one awful context, whereas the word retard has a wide frame of uses and references - although using it to degrade a person is wrong on every level. I think it’s very telling when a person uses these kinds of words. Apart from announcing to the world how much of a total fuckwit they are, it exposes their glaring feelings of inadequacy, and a severe lack of imagination.