In "The Lady or the Tiger?" what had the princess done which no other person had ever done?

The princess found out what was behind each door. 

In this kingdom long ago, a semi-barbaric king has an unusual system of justice.  He has built a huge arena, and outfitted it with two doors.  When a person is accused of a crime, he is thrown into the arena and forced to choose a door.  Behind one door is a lady, whom he will marry, and behind the other is a tiger, which will eat him. 

In the king’s mind, fate is really determining the innocence or guilt of the subject.  Fate is the jury.  If the subject is innocent, he will immediately be rewarded with a wedding to a suitable lady (whether he is attached or not).  If he is guilty, he will immediately be punished with a horrible death when the tiger mauls him. 

There is a problem.  The problem is that his daughter, the princess, takes a lover he does not approve of.  The easy way to get rid of the lover is to toss him into the arena.  Either way, this solves the problem.  Either he will die immediately or he will be married to someone else. 

However, the king’s daughter has other ideas.  She has a semi-barbaric streak too, and she is also wily. 

Possessed of more power, influence, and force of character than anyone who had ever before been interested in such a case, she had done what no other person had done—she had possessed herself of the secret of the doors. 

Through bribes and the strength of her personality, the princess is able to determine which door has the tiger and which has the lady.  However, this does not tell the reader everything.  Which door will she lead her lover to?  He knows that she has found this information out.  What the reader does not know is whether she wants him to live, or if she would rather see him dead than with another woman. 

Her decision had been indicated in an instant, but it had been made after days and nights of anguished deliberation. She had known she would be asked, she had decided what she would answer, and, without the slightest hesitation, she had moved her hand to the right. 

The story ends ambiguously.  We have no idea which door she chose. The author does this on purpose, because he wants us to ponder human nature.  Just how selfish is the princess?  I have always suspected that she chose the tiger.