How does Huck react to Buck's description of the feud? Does he understand? What does he think about the killing? Does he judge Buck's attempting to murder Harney Shepherdson?

I don't think Buck quite understands the reasons for inflicting such violence. Huck seems to have his own moral code that defies Southern tradition. Check out the following GradeSaver link,

http://www.gradesaver.com/the-adventures-of-huckleberry-finn/q-and-a/what-does-buck-say-when-huck-ask-how-the-feud-started-between-the-shepherdson-and-the-grangerfords-got-started-73871/

oops, I meant I don't think Huck quite understands such violence.