Albert Barnes Commentary 2 Samuel 15:14

"And David said unto all his servants that were with him at Jerusalem, Arise, and let us flee; for else none of us shall escape from Absalom: make speed to depart, lest he overtake us quickly, and bring down evil upon us, and smite the city with the edge of the sword." — 2 Samuel 15:14 (ASV)

And smite the city - David’s kind nature induced him to spare Jerusalem the horrors of a siege and the risk of being taken by assault. He had no standing army to resist this sudden attack from such an unexpected source. Possibly he also remembered Nathan’s prophecy (2 Samuel 12:10–12).