Why did Charlie stop the second rape?
Did he actually feel a bit guilty about previously letting his friend rape her and now he wanted to kill her husband and take ownership of her, thus he saw his friends rape as infringing on his property?
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I just saw the film and this called my attention as well. I think that in his twisted mind he thought that the first rape was just normal sex, and also that he didn't dare to stop his friend.
As for the second rape, I agree with you. He probably thought that her husband was pretty much gone and she was already his property.