Jealousy is a wicked emotion

Yes. Jealousy is a wicked emotion. So wicked, in fact, that it would overcome life in this battle. The princess cannot bear to lose her love to another, and so she would rather have him die. This is selfish, but jealousy is the most difficult emotion to combat. There is no direct opposite... at least not one I can think of, making it hard to stop once one falls victim to it. The author spends a great deal of time discussing how much the princess hates the woman behind the door, and that she had previous worries about her lover and the woman potentially getting involved. Unfortunately, the man would probably not expect this of the princess, and would trust her one hundred percent...and would walk to his death.