Throughout Huckleberry Finn, Twain is satirizing pre-Civil War America, a country whose Declaration of Independence directly stated that “all men are created equal” yet whose laws allowed Black people to be enslaved for nearly 100 more years. In particular, Twain is satirizing the hypocritical beliefs of Christians in the American South who felt comfortable judging others as “sinners” while supporting the violent institution of slavery. Twain’s satirical intentions are hinted as in a subtle way in the farcical “Notice” at the beginning of the novel:
NOTICE: Persons attempting to find a motive in this narrative will be prosecuted; persons attempting to find a moral in it will be banished; persons attempting to find a plot in it will be shot.
Given that it is impossible for an author to assess whether readers are attempting to find a motive or moral in the book, Twain is making it clear that he is trying to make some sort of moral claim beneath the novel, and he's winking at readers in the process. Twain makes it clear through moments of situational and dramatic irony throughout the book that this “motive” is to criticize the hypocrisy of those who supported slavery.