The Grangerfords and Shepherdsons don't even remember why their feud started, yet their sense of "honor" dictates that they continue it, going as far as murdering members of the other clan. Twain clearly finds something deficient in these families.
What these families call chivalry is false pride; what they call honor is hatred. These families are not at all honorable; they misguidedly equate hatred with honor and murder with justice. The two families attend church together, carrying their firearms with them, while the minister preaches about love. The hypocrisy is plain to see for the reader, but it's clearly lost on the families involved. The Grangerfords and Shepherdsons hear the sermon and even discuss it on the way home, but the essence of the message clearly eludes them, because they go back to their feud. In the Grangerford-Shepherdson feud, one might see Twain's condemnation of the false honor of the Southern way of life and of slavery. The church sermon holds the answer to the problem: love.
Go back and look for more evidence of this moral of the story of the Grangerford-Shepherdson feud. Add nuance and detail based on your own reading and analysis of the evidence in the pertinent passages.