Paul writes to Timothy (2 Timothy 3:16) that “all Scripture is God-breathed and profitable…that the man of God may be complete, thoroughly equipped for every good work.” In some passages of Scripture, it’s a bit challenging to find the profit. Take, for example, Psalm 137, which ends with “O daughter of Babylon, who are to be destroyed, Happy the one who repays you as you have served us! Happy the one who takes and dashes Your little ones against the rock!”
What do we do with this? One popular approach is to skip brusquely to “we can’t apply this literally, so let’s make up something edifying” as this author has done. I’d like to propose something better: something that starts with taking the psalm seriously in its original context.
Israel is in Babylon, having been brutally conquered and dragged into captivity. The psalm is a lament posing a question: how do they worship in a foreign land? This is not a simple question. The musical service of Zion was originally designed to serve as a parallel at David’s Tabernacle to the Tabernacle sacrificial service at Gibeon. At Solomon’s Temple, the musical and sacrificial services were brought together. (There are ascension offerings and ascension psalms, and so on.) With the Temple destroyed and the sacrifices no longer happening, was it even appropriate to sing the songs of Zion? (The editors who arranged the Psalter set it up so that the following songs answer the question posed in Psalm 137, but that’s a topic for another day.)
As they grapple with the question, their captors are demanding that the musicians sing songs of Zion purely for Babylonian amusement. Can you imagine? You’re a Levite, a son of Korah, your whole life devoted to sacred music in the Temple. All of a sudden, it’s all destroyed, and you’re a slave, and your master demands that you play sacred Temple music for the amusement of his guests at a drunken pig roast. That’s what Israel’s sacred musicians are facing.
And so the psalm closes with a curse on Babylon, and a blessing on the conqueror who does to Babylon what Babylon did to Judah. It’s not hyperbolic language; it’s a literal curse. It quite likely came to pass in the days of Belshazzar, with Darius’ Persian troops receiving the blessing.
So that’s what’s going on. After the cross, applying such a thing is complicated. You don’t get to curse your enemies and just say you’re following the example of the psalmist; the cross really did change some things. Today, we face strong counterexamples.
Jesus did the exact opposite of this curse on the cross (“Father, forgive them, for they know not what they do!”) Stephen followed His example (“Lord, do not charge them with this sin.”) James charges his readers that blessing and cursing ought not come from the same mouth (Jas. 3:8-12). In tension with that, Jesus Himself pronounced judgment on Jerusalem (Mt. 23:33-39), Peter cursed Simon Magus (Ac. 8:20-23), Paul blinded Elymas (Ac. 13:9-11), and asked God to repay Alexander the coppersmith (2 Tim. 4:14). Of those, two were clear means to the end of furthering repentance (Simon Magus) and the gospel (Elymas), and both had the desired effect (you could also put church discipline in this category). Matthew 23 arguably works this same way, given how it ends in v. 39, although we haven’t seen the fulfillment yet. Paul’s treatment of Alexander the coppersmith is less clearly redemptive, but notice that Paul does not specify what should happen to him, instead leaving him in the hands of the Lord to judge.
Where does that leave us? Before the cross, cursing your enemies was just common sense. Afterward, not so much. The Old Covenant is dead, and under the New, even the curses have a redemptive purpose. We are not allowed to simply follow the example set in Psalm 137; instead, we are called to follow Stephen’s example instead. Or Peter’s, cursing redemptively. So it is the easiest thing in the world to (in practice) just scrap the psalm–for all practical purposes, to mentally remove it from the canon of Scripture. “It’s not applicable today,” we say, and that’s that.
This is precisely where the ancient church comes to our rescue. Rather than simply discarding the psalm as an artifact of its time and place, inscripturated for some reason but utterly inapplicable today, the ancient interpreters take Paul at face value: *all* Scripture is God-breathed and profitable. The goal is not to figure out what we can disregard as “not applicable today,” but to wring every last bit of transformation out of our encounter with the Scriptures that we can get.
So where is the profit here? If we may not have this hatred toward our enemies, the ancient interpreters ask, is there something, some enemy, that we *should* have this hatred toward? Of course there is. “If you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live” (Rom. 8:13). We really should cultivate this antipathy — not toward people, but toward sin. The psalm really should resonate with us, not because we beg God to slaughter our enemies’ children, but because we call on God to destroy our sins.
Even the little ones.
Full Contact Christianity 
I would add that “daughter of Babylon” (137:8) may well be a Hebraic metaphor for anything in our lives that is tainted by the influence of Babylon so that her “progeny” in our lives are subject to extermination from our lives. This would certainly apply to living in our own present culture, saturated as it is with the influence of Babylon.