Wow, I literally just wrote a paper for school about this (yay me!).
Twain used the language that he did in such a way that would accurately reflect its usage in Mississippi during that time period. By today's standards, things like the "n-word" and the term "Injun" are considered extremely rude and ignorant. But it's usage in the book emphasizes the sacrifice Huck was willing to make for his black friend Jim. Despite the fact that everyone in his world viewed black people as sub-human, Huck chose to stand up in the face of hatred for the sake of love and friendship, knowing it would make him an outcast and could even result in him being sent to hell for eternity. He learned not to care what society told him - he figured out how to make decisions and form opinions on his own. So when you take those words and soften them mainly by turning the "n-word" into "slave," the audience not only loses an accurate representation of the perspectives people once held (which can help prevent us from slipping back into those perspectives in the future), but they also don't get a punch in the feels that's nearly as impactful as Twain intended it to be.