juliet=spy
If she knows so much that she would have to kill Sayid if she told him everything, then she has been invested in, programmed, made to be a team player, whatever you want to call it PLUS she was involved with Ben (however many different ways you want to interpret that!) whose favorite pasttime is not simply asking Michael to ask Jack to do some surgery, or even just asking Jack himself, maybe share a little cold chicken and play a game of foosball with him, nooooooo.... there has to be all this subterfuge and layers of manipulation involved, and why? Chain herself to Kate... if she wants Jack, why didn't she chain herself to Jack instead (why did Kate and Juliet wake up sooner than Sayid and Jack?). Because she is a spy and already knew stuff from Jack. Time to move on to the next person, and who is the easiest mark... the one who is close to Jack - that way you can use Jack against her... oh!... wait.... Juliet did... with the whole bear cage incident.

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