Both the Malbim and the Vilna Gaon write that Haman was bewildered because he had run out of an alternative, his Plan B. If Esther were not there, he would have emphasized the evil of the Jews, but Esther was there. If Achashverosh were not there, Haman would have told Esther that he did not know that the decree would harm the Jews. The presence of the both of them made any such excuse impossible.
R’ Moshe David Valle writes that Haman really had nothing to fear; he did everything only with the king’s permission. Although the king seemed to have forgotten, Haman did not. This is why he became bewildered.