Bhishma's vow of celibacy did not remove him from marriage politics, it only changed his role in them. To provide wives for his half brother he went to the svayamvara at Kashi and carried off the three princesses Amba, Ambika and Ambalika by force, which was a recognised if aggressive form of marriage for a warrior. Amba told him she had already chosen the king of Shalva. Bhishma released her, Shalva refused her because she had been carried off by another man, and Bhishma could not marry her himself because of the oath.
Amba went to teachers and kings for redress, found none, obtained a garland from a god that no warrior would wear for fear of Bhishma, and finally undertook austerities for a death that would let her come back and kill him. She is reborn as the child of Drupada, raised as a boy and later exchanged sex with a yaksha, and comes to Kurukshetra as Shikhandi.
On the tenth day Arjuna fights from behind Shikhandi, and Bhishma will not raise a weapon against one he regards as having been born a woman. He is shot down and falls on the arrows. The epic thus makes his death the direct consequence of the vow that made him great: the oath that took the throne from him also took a woman's marriage from her, and she is the instrument that ends him. Nothing in the sequence is an accident, and the poem does not offer him an excuse.