Why do you think Bilbo gave the Arkenstone to Bard and the Elvenking?

Do you think it was right for him to give away when it isn't his to give in the first place?

Based on the Elvenking's predictions, how do you think Thorin will react when he finds out that the only thing he desperately wanted out of all the treasure lies in the hands of his enimes?
Do you think that he will be upset with Bilbo?