"Now Esau learned that Isaac had blessed Jacob and had sent him to Paddan Aram to take a wife from there, and that when he blessed him he commanded him, 'Do not marry a Canaanite woman,' and that Jacob had obeyed his father and mother and had gone to Paddan Aram. Esau then realized how displeasing the Canaanite women were to his father Isaac; so he went to Ishmael and married Mahalath, the sister of Nebaioth and daughter of Ishmael son of Abraham, in addition to the wives he already had." NIV translation
We remember, from chapter twenty seven, that Esau was so angry that Jacob had "stolen his blessing" that he had planned to kill his brother. He must have been watching him and overheard the conversation about getting a wife. We also remember that Esau was more concerned about wealth and things of the flesh than spiritual matters.
With this in mind, he decided to get back at his father by marrying a woman that Isaac and Rebekah would not have liked. In doing this, we are reminded, once again, of the family of Ishmael and how he was born out of man's efforts instead of trusting in God. Esau married Mahalath which, in Hebrew, is Machalath and means "sickness". We also see that he already had other wives and so was definitely out of the will of God. This choice would have consequences for Esau but they continue to affect our world today.
God had promised Hagar that Ishmael would be a great nation but that he would not be a part of the chosen nation (see Genesis 21). Esau was a part of the chosen nation but was not selected by God to be the leader of that nation. He could have chosen to seek and accept the will of God in the matter but was not concerned about the things of God. This unholy union between the two families would constantly remind them of the fact that they were not the chosen ones. With this would come the "sickness" of disobedience and trying to overthrow the will of God which continues even to this day. Most of the strife that we see in Israel today, as well as in much of the region, is due to the fact that man seeks to overthrow the will of God for His chosen people and land.