Jamieson, Fausset and Brown on 2 Samuel 16:21

21. Ahithophel said unto Absalom--This councillor saw that now the die was cast; half measures would be inexpedient. To cut off all possibility of reconciliation between the king and his rebellious son, he gave this atrocious advice regarding the treatment of the royal women who had been left in charge of the palace. Women, being held sacred, are generally left inviolate in the casualties of war. The history of the East affords only one parallel to this infamous outrage of Absalom.