In "The Adventures of Huckleberry Finn," why did Twain include this adventure with the Grangerfords? (Chapter 15-18)

Twain used satire quite a bit.  Satire is when he made fun of people, ideas, movements, or anything, really, that he found preposterous, ridiculous, frustrating, idiotic, or illogically popular.  During the time period that this book was written, there was a real-life feud going on down in Kentucky and West Virginia, between two families called the Hatfields and the McCoys.  It started off with one incident, went back and forth, and even had a secret relationship between a girl and guy of opposite family ties, which led to a lot more bloodshed.  This feud got to be so violent and out-of-control that the national guard had to be called in to stop the craziness.  The feud was the gossipy fascination of the day; all of the papers covered it in detail, and people read the papers, reveling in each gory account.

So, naturally, Twain had to make fun of it.  He probably found the entire idea behind feuds ridiculous, the people in the feuds a bit backwards and hypocritcal, and people's fascination with such violence and bloodshed a bit alarming and worth noting.  So, he takes Huck into the middle of it in order to make fun of the type of people that got involved in feuds (note the southern family pride, the dramatic personality types, the overdone decorations, the gaudy tastelessness), and their hypocrisy (going to church with guns, calling on God to protect them in their idiocy).  He even includes a romance, just like the Hatfield-McCoy feud.  But then, Twain gets serious.  Huck is caught right in the middle of an all-out battle, and sees people shot to death.  The images so disturb him that he states of the affair:

"It made me so sick I most fell out of the tree...I wished I hadn't ever come ashore that night to see such things.  I ain't ever going to get shut of them-lots of times I dream about them."

Twain uses Huck to portray the more disturbing side of these feuds-the senseless killing and horrible violence and tragedy.  Huck, who is upset by very little (he even faked his own gory death for pete's sake), and brushes most things off in order to go with the flow, is so disturbed by what happened that he got sick, and still gets nightmares from it.  So, Twain uses the Grangerfords to make fun of the idiotic people involved in feuds, and to comment on the serious nature of such violence.