What is the Torah telling us here about Esav? Why did he marry the daughter of Yishmael?
Rashi: It is telling us that, when Esav saw how Yaakov carried out his father's instructions, and went to Charan to avoid marrying a daughter of Kena'an, he put on a pretense of fulfilling the wishes of his father by marrying a daughter of Yishmael. The trouble was however, that he did not divorce his wicked Kena'ani wives.
Rashbam: When he saw that his father had conferred upon Yaakov the Berachos of Avraham, and that he instructed Yaakov to marry a woman from his family and not from Kena'an, he figured that he lost the Berachos because he had married a Kena'ani woman, and that, perhaps, by marrying a daughter of Yishmael, he may regain part of the inheritance that he had lost. 1
Ohr ha'Chayim: He saw that Yaakov went to do two Mitzvos - Kibud Av v'Em, and taking a wife - therefore he would not be able to kill him. Even so, he sent his son Elifaz to kill him; he thought that perhaps his son can do what he cannot.
Ha'amek Davar: He saw that Yaakov would fulfill not only his father's command against Benos Kena'an, but also follow his advice to marry Lavan's daughter. He realized that his wives are unpleasing to his father.