Psalm 137:
8 O daughter of Babylon, doomed to be destroyed, blessed shall he be who repays you with what you have done to us!
9 Blessed shall he be who takes your little ones and dashes them against the rock!
Modern people have a difficult time handling the anger in the Psalms. Eugene Peterson writes, "People who are looking for a spiritual soporific don’t pray the Psalms, or at least don’t pray them for very long....."
There is a realism in how the Psalmist processes his anger. He takes his feeling and he opens them up in all their reality and looks at God in all of his reality. His emotions actually drive him to God in a new way, and His presence changes his emotions. Religion tends to say, stifle your feelings, especially those negative feelings. Get on top of them with goodness. You are a good person. Secularism tends to look at venting, and expressing your feelings, as an end in itself. The Psalms doesn't say, stuff your feelings or ventilate. It doesn't say stifle your feelings or bow to them. It doesn’t say be under-aware of them or over-aware /awed by them. You PRAY them. You let your emotional reality drive you to the reality of God and if the reality of God is true it ought to change your reality.
One more thing:
Jesus weeps the city as he heads into Jerusalem for the last time. Psalm 137 is on his mind. Little do the people know that this Psalm prophecies destruction of their city [which was fulfilled in 70AD]. But there's more on the mind of Jesus. He himself is to be 'dashed on the the rocks' FOR us.
What does this teach us? Anger demands justice from a just God. The cross, however, shows us more than a just God who exacts payment. It shows us a just God who pays for it. He took the judgment upon himself by becoming sin. The Father had dashed his little One against the cross for the sins of all humanity.
Anger on this side of the cross goes beyond demanding justice. It pays for the injustice owed - paying 'grace' forward. It wills the good of our enemies. Jesus did from the cross for us. "Forgive them," he prayed.