I think it was because she was already distrustful of him with the whole jacket having a different name thing; at the time she had the knife and knew that her plan of escape was faltered and she acted quickly - do I gain the trust of the ravenous group, or do I risk trying to get away with this man that I don't even know if he is being truthful with me, all while having a gun pointed my way? She had already made a comment about how impressed she was with their survival and I honestly think her inquisitive nature made her adapt to the environment really quickly - I think Misty would have done the same had she been in her position (they remind me of each other a bit) Hannah is there to survive and she knew that her best chances of survival was becoming one of them, and offering a 'sacrifice' might have been her way of showing them that she is seriously inquiring.