I disagree about Theon. He's always seemed a selfish and egotistical type. He may like and even admire the Starks, but he was always second fiddle with them, if not six or seventh. Plus Ned kept him humble. Combine the facts of Ned being gone, his egotism and the emotion of being home again and his quick betrayal is not surprising.

I'm not sure where people get the idea that Theon is one of the "good guys." I never got that impression from the books.