Charles Ellicott Commentary 2 Samuel 16:21

"And Ahithophel said unto Absalom, Go in unto thy father`s concubines, that he hath left to keep the house; and all Israel will hear that thou art abhorred of thy father: then will the hands of all that are with thee be strong." — 2 Samuel 16:21 (ASV)

And Ahithophel said. —The counsel of Ahithophel was in effect that Absalom should make the breach between him and his father absolute and irreconcilable. His followers would thus be assured of the impossibility of his securing a pardon for himself while they were left to their fate. After adopting this course, he must necessarily persist to the end. The taking of the harem of his predecessor by an incoming monarch was an Eastern custom, to the enormity of which the mind was blunted by the practice of polygamy.