What is the meaning of "Kol ha'Am Nadon"?
Rashi, Targum Yonasan: They were debating with each other.
Malbim: Everyone said to one another.
Since David saved them, why did they prefer Avshalom when he was alive?
Malbim: Indeed, this was returning bad in place of good, and foolish, for Avshalom had not saved them!
Above (15:3-4), Malbim explained that Avshalom told everyone who came for judgment 'the king will not hear your case, due to old age, laziness, or he cares only about his own Shevet. I wish I were Shofet, to remove stumbling in sin and vindicate people!' It seems that people spread the word; he deceived the hearts of [his father, Beis Din and] Yisrael (Sotah 9b)! (PF)
What is the meaning of "me'Al Avshalom"?
Radak: [He fled him] lest he oppose him in war.
Radak citing Targum Yonasan: From being with.
