Ahithophel - It has been with great probability supposed that Ahithophel was estranged from David by personal resentment for his conduct in the matter of Bath-sheba and Uriah (see 2 Samuel 11:3).
While he offered sacrifices - Rather, that Absalom sent for Ahithophel to be present when he offered the sacrifices; the intention being that all who partook of the sacrifice should be bound together to prosecute the enterprise. Absalom, too, would take advantage of the excitement of the great feast to inflame the ardour of the guests, and pledge them irrevocably to his cause.