My gut reaction was Sun. But then I thought "was she villainous in this episode?" And, no I don't think she was; she didn't deny what she had done, she did not try and justify it. She had even accepted that Jin had every right to leave her, and were it not for Jin acknowledging what he had done to push her a way then I don't think Sun would have been wrong...
So that leaves Juliet and the Captain. Although Jin deserved to know about Sun's affair, Juliet (in my opinion) did not have the right to tell him. Even though I do think she did it for Sun (and the babies) best interests, I'm going to vote her because I don't think the Cap'n did anything villainous.... yet
