Despondency
By comparing 2 Samuel 11:3 and 23:34, some believe that Ahithophel was the grandfather of Bathsheba, and that he was enraged because David had committed adultery with her. So Ahithophel may have wanted Absalom to destroy David and take the throne from his father. But when he saw Absalom following advice that would lead to defeat, he was so despondent that he gave in to despair and took his own life instead of committing the matter to God.