Charles Ellicott Commentary Jeremiah 41:9

"Now the pit wherein Ishmael cast all the dead bodies of the men whom he had slain, by the side of Gedaliah (the same was that which Asa the king had made for fear of Baasha king of Israel,) Ishmael the son of Nethaniah filled it with them that were slain." — Jeremiah 41:9 (ASV)

Because of Gedaliah. —Literally, by the hand of Gedaliah; i.e., by using his name to entrap the unsuspecting pilgrims.

Which Asa the king had made for fear of Baasha ... —See 1 Kings 15:22; 2 Chronicles 16:6. Baasha had tried to fortify Ramah as an outpost of his kingdom. Asa called in the help of Benhadad, king of Syria, and compelled him to desist, and then carried off the stones and timber to strengthen Mizpah as a position of defence. The “pit” was probably a trench with a drawbridge over it, so constructed as to stop all approach from the neighbouring kingdom; or else one of the tanks or reservoirs constructed to supply the fortress with water. A various reading gives “the pit of Gedaliah,” instead of “because of Gedaliah.”