Bilbo´s treachery

[Inziladun]

Originally Posted by Mithadan

But candidly, the language used by Tolkien suggests another explanation. "Bilbo's arm went to it drawn by its enchantment." One can also refer to the poem The Hoard found in Adventures of Tom Bombadil where he discusses the power and bewilderment of treasure. The original title of this poem (it was first published in 1923 in a Leeds University magazine) Iumonna Gold Galdre Bewunden, meaning Gold of Men Enmeshed In Enchantment (See Annotated Hobbit, p. 288). This is a recurring theme in Tolkien's works, the love of treasure as a motivator towards either evil or bad decisions. I tend to favor this explanation of the Arkenstone incident.

I think back to the time at Bag End where the Dwarves sang their song before setting out.
There, Bilbo listened and was "enchanted" to some extent, sharing the love the Dwarves had for jewels and such, out of character for most Hobbits in itself.
He'd had much time since then to hear the Dwarves talking about all that treasure, and had also spoken with the dragon who stole the hoard. I favor those things as influencing Bilbo's behavior.