All of that has to be brought over in that book. Eliminating it is to make the slaveowners look like nice guys.
That's not entirely true. Many of the founders were slave owners through inheritence but were repulsed by slavery and fought it. One of them set his slaves free which resulted in laws being passed that prevented the others from freeing theirs.
Slavery was not a ... pardon the pun... black and white issue.
I guess that means virtually the whole country was disgusting, since they all used that word for blacks. That's what it means—black, coming from the Latin niger, by way of various Romance languages. Usage changes with time and fashion, and reading books in their original words, such as Chaucer, Milton, and Shakespeare, is one way schoolkids learn this.
Twain's usage—and note that he was a big liberal—is not a reason to put Wite-Out over literary history. Look at the barbed humor kids would miss, from Tom Sawyer: Here Tom is describing a horrific steam boiler explosion he witnessed. AUNT POLLY: Was anyone hurt? TOM: No ma'am. Killed a n______. What are the kids supposed to do when they read any other books published more than 5 minutes ago, and discover how the polite word for blacks has changed frantically just about every decade for a half-century? Just in my lifetime: colored, Negro, Afro-American, black, African American . . .
If the great evils are demonized, how are Leftists supposed to relive them without resistance?