We do not have to take the narrator's word for it that Thorin is becoming
possessive. Before laying eyes on the gold he offered Bilbo his pick of
the treasure; now (despite the fact that he played no part in killing Smaug) he
says he will be avenged on anyone who withholds the Arkenstone. Thorin is
becoming possessive, and threatening his companions, and regardless of his
legal rights, possessiveness is a moral failure in Tolkien's world. I
don't think Thorin is swearing a formal oath here,
though.
“I dislike Allegory - the conscious and intentional allegory - yet any attempt to explain the purport of myth or fairytale must use allegorical language. (And, of course, the more 'life' a story has the more readily will it be susceptible of allegorical interpretations: while the better a deliberate allegory is made the more nearly will it be acceptable just as a story.)” (From Tolkien Letter # 131.)
