I am still of the mind that Juliet and Ben are working together as a team to break Jack down. They are working quite effectively to get him to lean on Juliet to some degree and trust her. Why should he believe her when she says Ben is a liar or that he is dangerous? I think she is telling him what he already believes in order to form common ground with him. I agree that she is quite dangerous, and from Sawyers observation would have killed Kate without blinking....that cannot be good.