There is always something special about the moment when a reader has been ahead of the plot and then catches up. What happens next is when the ability of the storyteller really comes through. If it becomes easy to guess then to some degree the story has failed but equally go too far from reality and it becomes implausible.
Sagan has now reached that moment. It has become clear that Lewis is killing anyone who tries to insult or hurt Dorothy. Finally the penny drops and she confronts him with the truth and he admits it easily enough. But then she is in a dilemma because she realises the power she has over life and death – is she dislikes someone then Lewis will happily kill them for her.
She keeps it secret to herself but you wonder just how lone she can go on living in the shadow of the strange young man who is a cold-hearted killer.