Albert Barnes Commentary 2 Kings 22:20

"Therefore, behold, I will gather thee to thy fathers, and thou shalt be gathered to thy grave in peace, neither shall thine eyes see all the evil which I will bring upon this place. And they brought the king word again." — 2 Kings 22:20 (ASV)

In peace - Josiah's death in battle (2 Kings 23:29) seems to contradict this prophecy verbally, but it does not oppose its true spirit. The prophecy simply meant that the pious king, who had sent to inquire of the Lord, would be gathered to his fathers before the troubles that would bring utter desolation upon the land.

Those troubles were not to come from Egypt, but from Babylon; their commencement was not the invasion by Pharaoh Necho in 608 B.C., but the invasion by Nebuchadnezzar three years later. In this way, Josiah was taken away from the evil to come and died in peace before his city suffered an attack from its truly formidable enemy.