What does Huck witness while hiding in the tree in The Adventures of Huckleberry Finn? What is his reaction to what he sees?

While Huck is hiding in the cottonwood tree in Chapter 18, he sees the feud between the Sheperdsons and the Grangerfords come to life (or perhaps I should say to death).  He sees Buck Grangerford and a cousin of his fighting against a group of Shepherdsons.  He sees the two of them end up getting ambushed and having to jump in the river while the Shepherdsons are yelling about killing them and shooting at them while they tried to escape.

Huck is revolted by what he has seen.  He says that he will never be able to rid himself of the images.