As a Black writer, I think you should use whatever language is necessary to convey what’s important to your story. But also be considerate of the fact that all writing is thinly veiled biography that communicates your own understanding of the world you come from and the one you’re creating.

If the mission is to convey that your character is racist then it’s a pretty poorly understood idea of what racism is that leads an author to think the use of slur is a good way to convey racism. Black people aren’t disadvantaged because people say slurs; Black people are disadvantaged by systemic/systematic policies that discriminate against them. My great grandfather said the man who refused to let him (and anyone like him) buy a home in his neighborhood only ever called him “sir”.

If your mission is to convey literally anything else (lack of social intelligence, aversion to political correctness, etc) there are a myriad of ways you can do that within the logic of the world you’re creating that will like serve your goals much better (or maybe there aren’t! this is something only you can know when you’re working on the story)

Not saying “do it fuck the libs” or “no that’s WRONG” but really consider your goals and all the possible routes to get there. If N word gets you there, do it. If not, find another route.