Tearing up the letter is significant because it shows that Huck has rejected the racist attitudes of society and decided to follow his conscience.
Throughout the book, Huck struggles with the idea of slavery. Before spending so much time with Jim, he never thought much about it. Jim shows him that slaves are real human beings with feelings and pain. When Huck tears up the letter, it is a symbol of how he is rejecting the narrow-minded hate of prejudice.
Huck is feeling bad about helping a slave escape. He has always been taught that it is wrong. There is nothing worse in Huck’s world than being an abolitionist. He writes a letter to Miss Watson to tell her where her escaped slave is, and at first he feels good, but then he feels bad. His really conscience, not what society tells him is his conscience, tells him that slavery is wrong.
“All right, then, I'll go to hell”—and tore it up. (Ch. 31)
This is one of the most significant lines in American literature because it demonstrates the American spirit of independence. There is nothing more important to Americans than being able to make up their own minds. Huck has done that. He has rejected the institution of slavery completely, and with it he is rejecting mainstream society.
Huck realizes what he is doing.
I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. (Ch. 31)
This is a brave thing to do, but it comes from the heart. Huck believes he will go to Hell for this, but he doesn’t care, because to him he is doing the right thing.
Although this is not an actual historical event, the publishing of the book is. Twain captured a national consciousness with his work, and a shift in perception. Slavery was becoming less accepted and more people opposed it as time went on, and Twain's purpose was to demonstrate that not every American blindly followed the crowd.