Well thats different because the context is very, very different. A white person calling a black person nigger out of anger and hatred is racism, as the word used in that context is used out of racism. If i call a newb a gimp or retard for making a silly post, the words used in this context are used to show theyre an idiot. Not disabled. Pretty fair difference.
Because the term now is unacceptable it is only usable in that way. It was once used casually the way gimp and tard are now used. Ignoring the use of words like tard and gimp don't allow them to become universally unacceptable.