Your theory is still wrong. Juliet may help the losties but for only her own personal gain. She is a self serving killer, just like the rest of the others.
with all due respect juggy, no ones theory can be wrong per say until proven, and the jury is still out on all this

with that being said, my thought is juliet is still a good soul deep down, i dont think she will kill any losties, unless her life is threatened, and i dont think she will do anything ben says because he is her ticket off the island.....its obvious that her little bob dylan skit was true, based on the little "i hate you ben", so if he is her ONLY ticket off the island why kill him??
now i think the reason she is playing both sides of the fence is to gain the losties trust, she cant just up and go against the others without having a definite trust with the losties, simply because she cant be on her own
i think she is helping Sun (and possibly Kate) because she truly wants to see her work actually work......i cant imagine her watching a pregnant woman die without trying to help no matter who it is, its not in her nature
so yes, she has done what she's needed to in order to survive, but to say she is a heartless cold killer is a bit much, i mean some losties have killed people themselves but it was out of survival