Juliet's situation isn't that she's benevolent or malevolent. That is one of the main themes of this show: no one main character is really good or bad...absolutely. All the characters seem to vacillate between being selfish and selfless. Juliet does "bad" things to the Losties because she's compromised by Ben's control over her stay on the island. Ben does "bad" things because he's compromised by Jacob and his loyalty to the island. Jack goes back and forth between his charismatic leader/savior side and selfish/paranoid side.

As for Juliet working for Ben still, I have my bet on him having control over her in some way or another. Remember that they set up a false trust between Juliet and the Losties before with making Claire sick. Incapacitating Jack with an unnecessary surgery (perhaps brought on by another poisonous agent hidden in food or water) could allow the Others via Juliet to have more control over the Losties's fate. It seemed to have worked in making her the de facto leader when Jack went down (as darnzy has already suggested).

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Abaddon