What is Twain saying in The Adventures of Huckleberry Finn about life in pre-Civil War Missouri?
Numerous answers are available to this particular question regarding this particular novel.
Twain suggests that life in Missouri (and further down the river) somewhat hypocritical due to a distinct religiously derived abstraction that infiltrated the character of the citizens. Following generalized and self-serving tenets of behavior and grounding these in religious justifications allows for the existence of slavery and rampant fraud.
This basic moral background also creates figures like Pap Finn and Tom Sawyer, figures of privilege who use their status as white males to inflict harm on others with impunity.
Life in this area is also quite provincial (removed from the sources of culture and cultural life). This trait is expressed through Huck's "educated" view of the kings of England and the comical mis-representations of Shakespeare's plays by the Duke.