Does everyone remember when Juliet hung up those cards telling Jack to kill Ben? Was she for real? If she was, then why did the others completely freak out when Jack was about to let Ben die? I'm so confused!

My only theory: Juliet is actually working for the "other" others, such as that one girl with the slingshot. It's very apparent that there are two groups of others, whether they are both on the same island, or on separate islands. Juliet could be a "double agent," so to speak.