Only Bilbo’s gift of the Arkenstone to Bard earns the word “betray”.  In this word, be- is an intensifier added to the obsolete verb “tray” – to trouble, vex, or afflict.  It took two thirds of the book for Bilbo to grow into someone close and trusted enough for his actions to be betrayal.

• 16.032 Are you betraying your friends,
• 17.019 I am betrayed,’

“betray, v.” OED Online. Oxford University Press, March 2015. Web. 6 May 2015.