What is Twain's message about Southern life as satirized by the feud in chapters 15-20 of The Adventures of Huckleberry Finn?
In my opinion, what Twain is satirizing here is the tendency in the South (especially before the Civil War) towards believing very strongly in ideas of personal honor that seem to have been taken from the romantic literature of people like Walter Scott.
In the antebellum South, there was the attitude that the rich were an aristocracy. They saw themselves as an elite who needed to (like the feudal aristocrats of Europe) spend a lot of time building their honor and protecting that honor. This is why the South had so many duels between gentlemen. I believe that Twain is satirizing this ideal.
Twain's message about Southern life extends to its traditions. By having two families fight a feud that they no longer know why, he satirizes that traditions are wrong if there is no purpose to them and if no one knows why they even exist, then the traditions should end. This was a subtle allusion to slavery, a less subtle one to European values and other traditions that the South continued.
It is also a satire of gaucheness in the descriptions of the households in the grand manner of English and French estate homes. The decor was tasteless and by adding a porrly worded ode, a European invention, he completes a satire of the ways in which people appeared to live beyond their means.