Baleran Cole

IMHO, most of all it's Robb's lines that convince him. Because as Robb cleverly reminds, it's treason to bare one's steel against one's lord. He could easily have the Greatjon executed (not very clever when you want your bannermen to go to war but it's his right). The North is an harsh place. We had occasions to see how the likes of Bolton do justice there. The loss of three fingers is better than the loss of one's life. The Greatjon is punished and offered in the meantime a way ouf of the mess he has put himself in by the "doubtless you meant to cut my meat" line.