Non-American here. I also didn’t get this, thinking it’s just puritanical bullshit. Some Americans seem obsessed with auto-censorship.
Anyway, I finally understood while watching Django Unchained. It’s an extremely dehumanising word, meant to separate people (who have rights) from things which do not. It’s a tool to be able to do this distinction and then do unspeakable evil to specific people because they don’t count as people and so it’s alright.
Now remember that slavery was ended* only relatively recently, segregation was a thing during the lifetimes of many people and this mindset of black people not being even human is still prevalent…
The word is meant to be always used in hostility and it’s still being used like that today. That’s why you want to steer clear of it.