We are each and all responsible for our own behavior, no matter what society says is "moral" or "right". This seems to be the message that underlays the novel and the point of much of the novel's satire. Huck is disturbed by the difference between society's mores and his own sense of loyalty, integrity, and honor.
I agree with others that Twain is really satirising so many aspects of Southern American society. The Grangerford incident really satirised the kind of feuds that existed between Southern families - note how both the Grangerfords and the Shephertons bring guns to the service and listen to the sermon, which is on brotherly love, and then go back, discussing the sermon, but obviously not letting it penetrate their hearts at all!
There are many targets of Twain's satire in this work. The hypocrisy of human nature is just part of it...he also makes fun of the organization of religion, of con artists and their gullible victims, of pride and vanity, and the lopsidedness of...
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