What event causes Huck to want to flee? Why does he feel guilty about what happened?

The next day Miss Sophia elopes with Harvey Shepherdson, and the feud is rekindled in full force. Buck's father and both his brothers are killed in an ambush, and Huck arrives at the harbor in time to see Buck and his cousin shooting at five grown men. Eventually the men manage to sneak around Buck and kill both the boys while Huck watches from a tree that he climbed in an attempt to find safety. Once the Shepherdson's have left, Huck pulls Buck and the other boy out of the river and onto dry land where he weeps and covers their faces.

Huck runs back to the house and sees that it is quite silent in the wake of the family tragedy. He goes to the swamp, finds Jim, who is glad to see that Huck lived through the massacre, and together they push the raft into the river and start floating downstream.