Who killed King Josiah at Megiddo?

Answer: Pharaoh Neco

Pharaoh Neco marched to the Euphrates to help the king of Assyria; Josiah went out to confront him, and Neco killed him at Megiddo.

“During Josiah’s reign, Pharaoh Neco king of Egypt marched up to help the king of Assyria at the Euphrates River. King Josiah went out to confront him, but Neco faced him and killed him at Megiddo.”— 2 Kings 23:29 (BSB)