How will having Bi'ah with David's Pilagshim strengthen Avshalom's supporters?
Rashi: Now they waver, for they fear lest Avshalom regret [and cease to rebel, and be submissive] to his father, and the king will hate them [for supporting his rebellion]. Now they will see that you despise your father.
Why did Achitofel give this counsel?
Yalkut Shimoni (151): He told Avshalom that Hashem decreed it - "Hineni Mekim Alecha Ra'ah mi'Beisecha
Kli Yakar: Avshalom could say, I did like you counseled me! It seems that in private he told Avshalom to do so. This is why it does not say that it was good in the eyes of the Chachamim, like it says about the counsel to kill David (17:4).
Why did he mention that he left the Pilagshim to guard the house?
Malbim: This shows that he still considered him like a son. One would not leave his Pilagshim 1 if an enemy pursued him, lest the enemy kill them or make them Hefker [to his men for Bi'ah]. Why does he need to guard the house? Surely Avshalom will take it! Rather, he is guarding his house for his son.
He took his wives! Perhaps he wanted their company. He would have taken also his Pilagshim, but he needed someone to guard the house. (PF)
