Albert Barnes 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 - By the side “of Gedaliah.” Ishmael now cast beside Gedaliah’s body those of the pilgrims.