John Gill Commentary Psalms 137:8

"O daughter of Babylon, that art to be destroyed, Happy shall he be, that rewardeth thee As thou hast served us." — Psalms 137:8 (ASV)

O daughter of Babylon, who are to be destroyed
By the determinate counsel and decree of God, and according to divine predictions; see (Jeremiah 50:1-51:64) ; so mystical Babylon, antichrist, and the man of sin, who therefore is called the son of perdition, (2 Thessalonians 2:3) ; because appointed to destruction, and shall certainly go into it, (Revelation 17:8) ; or "O you destroyer", as the Targum, which paraphrases it thus, "Gabriel, the prince of Zion, said to the Babylonish nation that spoils or destroys;" which is true of literal Babylon, called the destroying mountain, (Jeremiah 51:25) ; and of mystical Babylon, the destroyer both of the bodies and souls of men, (Revelation 11:18) ;

happy [shall he be] that rewards you as you have served us ;
meaning Darius the Mede, as Kimchi; or rather, or however who must be added, Cyrus the Persian, as R. Obadiah; who were ordered by the Lord to retaliate her, and do as she had done to others, (Jeremiah 50:15Jeremiah 50:29) ; and in so doing pronounced happy, being the Lord's shepherd, raised up in righteousness to perform his pleasure, (Isaiah 44:28) (45:13) ; and here wished success by the godly Jews. In like manner the Christian princes will reward mystical Babylon, and be the happy instruments of her ruin, (Revelation 18:6) .