Albert Barnes Commentary Jeremiah 21:9

"He that abideth in this city shall die by the sword, and by the famine, and by the pestilence; but he that goeth out, and passeth over to the Chaldeans that besiege you, he shall live, and his life shall be unto him for a prey." — Jeremiah 21:9 (ASV)

He that ... falleth to the Chaldeans - This was to counsel desertion, and would have been treason in an ordinary man: but the prophets spoke with an authority above that even of the king, and constantly interfered in political matters with summary decisiveness. .

A prey - Something not a man’s own, upon which he seizes in the midst of danger, and hurries away with it. So must the Jews hurry away with their lives as something more than they had a right to, and place them in the Chaldean camp as in a place of safety.