Theon Greyjoy is a despicable character BUT one has to look at it from his point of view. He is an oath breaker and murderer but at the same time he is trying to remain loyal to his blood family who clearly despise him.

It's an interesting parallel with Tom Hagen, of all people, because both men were raised as "adopted children" by more powerful families and each of them knew they could never hope to inherit power from their adopted family. The difference though is that Hagen's family is gone and never had any conflict with the Corleones.

Theon's family is still very much around and has major unsettled business with the rest of Westeros and most especially the Starks. It was foolish for Robb to send Theon back to his family but was in keeping with the "honor before reason" characteristic which seems to plague Stark males. Once Theon's father decided to attack , Theon was in an impossible position. There was no way he could turn against his own blood. I thought Alfie Allen did a really good job portraying Theon's internal struggles as well as his essential need to prove that he's as good or better than the Starks.

Ultimately one could blame Ned Stark's mercy in taking Theon as a ward all those years ago instead of just ending the Greyjoys completely, which is certainly what Tywin Lannister would have done. But hindsight is always 20/20.