During my time as a college English student, with professors being predictably as you can imagine they were, I was struck by how powerful a set of letters “nigger” could be — first, as a dehumanizing attack and, later, as a literary marker of the speaker’s ignorance. Particularly in postbellum literature, and especially in certain fonts, that double-g looks like a dark jab scattered across the page. Whether the book that first gave me that impression was something by William Faulkner or was Huck Finn, I don’t recall, but it came to mind upon reading of an edition of Mark Twain’s book that replaces all instances of the word with “slave.”
As Twain once said, “the difference the almost right word & the right word is really a large matter — it’s the difference between the lightning bug and the lightning.”
Rich Lowry has posted a letter that makes the point well. Readers of Huck Finn can’t help but discern the author’s criticism of those using the word, and the callousness of their attitude toward human life. There’s a callousness to removing the word, as well.
My reading of the book, which I described in academic detail in the essay that kept me out of Brown University’s graduate program in Literature, makes this point central to a sly, more intriguing intention that I believe to have been Twain’s underlying purpose for the book. Addressing longstanding and heated disapproval of Twain’s reintroduction of Tom Sawyer for the climax of the book — which has led multiple critics to declare the ending an unforgivable failure, with no less a figure than Ernest Hemingway calling it “cheating” — I proposed that Twain was putting the reader in the position of the character of whom he or she was apt to be most critical:

When it is considered that, at Huck’s moral juncture, Tom comes into an adventure in progress with privileged information, a new link is seen: this time to the reader. Tom’s reappearance for the Phelps section does lead to a change in the book (as is evident from the controversy over the end), but only inasmuch as we were expecting (read “hoping”) for something different. A reader hoping to read a Jim-as-hero-escaping-from-slavery story would be, essentially, hoping to do (or hoping that the author does) exactly what Tom tries to do from his point of view: make the book interesting in a certain way, in part by making Jim into a specific type of hero. In the Connecticut of the 1880s, this would translate into a desire to “set [Jim] free” even though “he was already free” (Twain, 262). It is not necessarily requisite to this conclusion that the reader of this, or our, era would see Jim, specifically, as free; it is enough that the post–Civil War reader (and, more so, the modern reader) would consider freedom as some intrinsic quality of humanity in much the same way that it is possible, now, to see the Emancipation Proclamation as an overdue formality — the Civil War itself can be said to have set free people free (like a liberation of civilian hostages in a hostile country who are being held unjustly or against their rights). …
Ultimately, a reader who is upset at the ending is put in a parallel role to Tom — wanting to set a free slave free in a manner that accords to his or her own sense of heroism (and, if you wish, morality). As stated by Fritz Oehschlaeger, “something in us longs for quite a different outcome, one that would allow Jim to retain his heroic stature and force Huck to live up to the decision that accompanies his tearing up of the letter to Miss Watson.” [21] In other words, like Tom, the reader wants circumstances to allow Jim and Huck to become heroes according to the reader’s definition.

It is not a testament to a fortitude of national character that a significant portion of our population would, in a sense, so dramatically merge the reader’s role with that of Tom. To my reading, Twain merely implied the connection, in keeping with his dark, wry humor. Now, in seeking to sanitize the culture that enslaved Jim, making the story more to the tastes of the modern audience, the reader is doing precisely what Tom Sawyer has drawn fire for doing: selfishly making light of the black man’s predicament.