Ps 136:8

“And in this way also the just give up to destruction all their enemies, which are their vices, so that they do not spare even the children, that is, the early beginnings and promptings of evil. In this sense also we understand the language of Psalm 137: "O daughter of Babylon, who is to be destroyed; happy shall he be that rewards you as you have served us; happy shall he be that takes and dashes your little ones against the stones." For "the little ones" of Babylon (which signifies confusion) are those troublesome sinful thoughts that arise in the soul, and one who subdues them by striking, as it were, their heads against the firm and solid strength of reason and truth, is the person who "dashes the little ones against the stones"; and he is therefore truly blessed. God may therefore have commanded people to destroy all their vices utterly, even at their birth, without having enjoined anything contrary to the teaching of Christ. And he may himself have destroyed before the eyes of those who were "Jews inwardly" all the offspring of evil as his enemies. And, in like manner, those who disobey the law and word of God may well be compared with his enemies led astray by sin; and they may well be said to suffer the same fate as they deserve who have proved traitors to the truth of God.”

“And David, pitying her, says, "O wretched daughter of Babylon." Wretched indeed, as being the daughter of Babylon, when she ceased to be the daughter of Jerusalem. And yet he calls for a healer for her and says, "Blessed is he who shall take your little ones and dash them against the rock." That is to say, shall dash all corrupt and filthy thoughts against Christ, who by his fear and his rebuke will break down all actions against reason, so as, if any one is seized by an adulterous love, to extinguish the fire, that he may by his zeal put away the love of a harlot and deny himself that he may gain Christ.”

“Then he turneth himself to her, "O daughter of Babylon, unhappy;" unhappy in thy very exulting, thy presumption, thine enmity; "unhappy daughter of Babylon!" (ver. 8). The city is called both Babylon, and daughter of Babylon: just as they speak of "Jerusalem" and "the daughter of Jerusalem," "Sion" and "the daughter of Sion," "the Church" and "the daughter of the Church." As it succeedeth the other, it is called "daughter;" as it is preferred before the other, it is called "mother." There was a former Babylon; did the people remain in it? Because it succeedeth to Babylon, it is called daughter of Babylon. O daughter of Babylon, "unhappy" thou! ...”

“Daughter. Citizens. — Miserable “plunderer,” (Symmachus) or “ruined,” (Aquila) or “which shalt be given up to plunder.” (Theodoret) — Cyrus reduced the city to a state of abjection, and it has since experienced other miseries, (Calmet) so that its situation is now unknown. (Haydock) — Isaias (xiii.) foretold this destruction. (Worthington)”