This psalm was written when the children of Israel were in exile. The Babylonians had conquered Jeruselem and the writer is expressing his anger towards the Edomites who were Israel's neighbors and cheered on the armies of Babylon. As he writes this verse, he is imagining the day that God repays them for what they have done. Pretty gruesome yeah? But it is in the scriptures none the less.
God has brought several verses to me recently, that have to deal with injustice. I admit that one of the hardest things for me to deal with sometimes is fairness or justice. Many times, I witness things that make me feel like the psalmist when he wrote earlier in this text. "But how can we sing the songs of Zion in a foreign land?" ( vs. 4 ) How can we be filled with the joy of the Lord, when we are oppressed by evil? I've been reminded lately, that that's just the way God works. When were we ever NOT in a foreign land?