Muso
He had already planned to choose her. He changed the last test because she did not pass the first two. He made that statement because it's a fact and that the nobles would agree and accept whomever he chose. He had already decided to kill the empress. The question is, why did he choose her? Is it her family is not as high as the others? Or is it something the family did in the past? (The father apologized to her when she was about to die....)