In the short story by Frank Stockton," The Lady or the Tiger", I think the princess's ultimate decision was to sacrifice her lover to the tiger. The beginning of the story talks about the barbarian rule under which she lived which I think affected her greatly. If she lets her lover live, she must forever watch him be with a beautiful woman she dislikes, and I don't think it is in her to be able to watch that for a lifetime. I think if she has to say goodbye, she will do it with his death.
Even though the princess lived under the barbarian law, the fact that she had a relationship with this man behind her father's back proved that she was openly defiant. The fact that once the lover was captured, she took it upon herself to find out what was behind the door, this showed her willingness of unhibited love toward this young man. The love she had for her lover made it easier for her to choose the woman behind the door because she couldn't live knowing she sent her lover to his death.