In the Deuteronomic curses, Moses informs the Creator God’s people that one of the curses that will come upon them for their failure to obey their God’s commands (of which David was certainly guilty) would be that “You will be engaged to a woman and another man will rape her” (Deuteronomy 28:30a). This is not strictly analogous to what Nathan has told David, or to what it has been suggested that Absalom do, as neither this God’s threat through Nathan nor Ahithophel’s suggestion to Absalom carries with it (at least on the surface) the connotation of violence or force, but it can probably be thought of as being connected closely enough to drive home the point to David that he has violated his God’s commands.
Additionally, Ahithophel apparently sees a close enough connection in that it will play well into Absalom’s ongoing effort to show himself as a true deliverer in the mold of Moses, thereby allowing Absalom to continue co-opting the most powerful story of Israel’s history for his own purposes. Furthermore, it adds to Absalom’s claim to be a just man and the one that is used by the covenant God to deliver justice to Israel. This is especially and strikingly so if Absalom is the means by which the prophecy (judgment) related to cursing that had been delivered to David by Nathan is fulfilled.
This merely cements the notion that David is no longer fit to be king, while also pointing to the fact that the story of Bathsheba and David, and the oppression and injustice that the story entails, has been made known in Israel. If it has not, then there is no real point in Absalom engaging in sexual relations with his father’s concubines, unless it is also being used to indicate that just as David has forsaken these wives of his, that he has also forsaken his care of the people of Israel as well.
The second part of Ahithophel’s response to the request to provide advice to Absalom is to say that “All Israel will hear that you have made yourself repulsive to your father. Then your followers will be motivated to support you” (2 Samuel 16:21b). Yes, Ahithophel suggests that this will be viewed by the people as Absalom being willing to be cursed by his very own father, if it indeed means justice for Israel. This is powerful symbolism. Absalom will be seen to be willing to bear that pain and shame on behalf of the people, with this becoming a messianic role. In a society based upon honor and shame, this is a calculated move (though also prophetically fulfilling) to win further sympathy from the people.
Does this aid Absalom in his desire to be seen as Moses? Absolutely! Moses was willing to forsake his father’s (Pharoah’s) house so as to identify himself with the people suffering under the oppression of the king. Thinking beyond that however, this might also be an attempt to entice David to retaliate against Absalom, who up to this point has not lifted up his hand (nor asked anybody else to lift up their hand) against his father.
David has left willfully. He has abandoned his throne and fled from Jerusalem and Absalom has peacefully entered to take that throne. There has been no bloodshed, violence, or loss of life (except Amnon many years prior, but that has only tangential bearing on the events at hand at that time). If David now turns and raises sword and spear against Absalom and his supporters, then David is most certainly to be likened to Pharaoh, who allowed Israel to depart from Egypt and from his oppression peacefully, but then had a change of heart and set out to recover the Israelites (and his power) by violent means.