"Daughter of Babylon, doomed to destruction, he will be happy who repays you, as you have done to us" (137:8). How do Christians read a psalm that ends like this?

First, what the psalm says. It begins with weeping by the rivers of Babylon: "By the rivers of Babylon, there we sat down. Yes, we wept, when we remembered Zion" (137:1). The one verse it speaks to the LORD is a request: "Remember, LORD, against the children of Edom in the day of Jerusalem, who said, “Raze it! Raze it even to its foundation!”" (137:7). Then it speaks to Babylon, and its last two verses are 137:8 and "Happy shall he be, who takes and dashes your little ones against the rock" (137:9). In Hebrew happy in both verses is ashrei, the word the WEB gives as Blessed in "Blessed is the man who doesn’t walk in the counsel of the wicked" (Psalm 1:1) (tap happy). The psalm doesn't say who he is, and the psalmist doesn't say he will act against anyone himself. Christians read words like these in two main ways, each resting on its verses, and these notes don't choose between them. Both agree that the one verse spoken to the LORD is 137:7, that 137:8-9 name no one who repays and don't say the psalmist will, and that 137:8 speaks of repaying Babylon "as you have done to us" (137:8). Christians who hold each view take both of these as words for them: "Vengeance belongs to me; I will repay, says the Lord" (Romans 12:19), and Jesus' "love your enemies… pray for those who mistreat you and persecute you" (Matthew 5:44). They differ on how 137:7-9 sits beside them. The chapter on Psalm 109 sets out the same two readings of that psalm's prayers, with a third that rests on how Acts reads that psalm (Acts 1:16-20); it isn't carried here.