Why did David decide to flee?
Radak: If we will not be in the city, Avshalom will not come to the city. If we would be here and he finds us in the city, we will not be able to escape; he will strike the city.
Malbim: Avshalom is ready for war, and will be saved with many soldiers. David did not know who is with Avshalom.
Why did he say "Pen Yimaher"?
Radak: This is the first thing Avshalom will do; he expects to find us here. Therefore, we should rush to flee.
Malbim: If Avshalom catches [David and his men] close to Yerushalayim, even if they can flee, Avshalom will destroy Yerushalayim, and claim that David's men did so, because they rebelled against him. It was still not known who follows David, and who follows Avshalom, to know who destroyed the city. And even if it would become know that Avshalom's men destroyed it, they would say that it is because they were fighting with David's men nearby, and the city was a refuge and stronghold for them. Therefore, David rushed to flee.
What is the meaning of "v'Hidi'ach Aleinu"?
Radak citing Rav, in Shochar Tov 3: He will judge us like an Ir ha'Nidachas 1 .
R. Yudan, in Shochar Tov 3: The Owner of the cup began Lehadi'ach (rinse) His cup. He will not budge until He cleanses it well. Once David saw that Hashem cleanses him of his sins, he began to sing Mizmor l'David (Tehilim 3). Radak - once the punishment began, I fear lest it finish us off.
Ha'Torah ha'Temimah citing Zayis Ra'anan: The verse ends, "he will strike the city by the sword", like the law of Ir ha'Nidachas (this is the only similarity to Ir ha'Nidachas. There was no connection to idolatry here. - PF)
