Is the Bible cruel?

“O Daughter of Babylon, doomed to destruction, happy is he who repays you for what you have done to us – he who seizes your infants and dashes them against the rocks.” Psalm 137:8-9
A number of years ago I stumbled across an online Bible quiz. As I worked through the questions, it became immediately obvious that this was not a trivia game, but someone’s attempt to discredit the Bible by pulling out apparent contradictions, and verses suggesting that God is vicious and cruel. This was one of those verses, which inspired the question, ‘How should we feel about dashing babies’ heads against rocks?’ with the ‘correct’ answer being ‘happy’.
However, there is much more to be understood about this verse. The Babylonians had mistreated the Israelites and their women and children during the sieges which saw the vast majority of the people who survived led away into captivity. The psalmist was one of those taken captive, and is writing concerning his feelings for Jerusalem and his nation. Although he was mourning the loss of his homeland and his city, he consoled himself in the prophecies by Jeremiah et al. that Babylon would one day be destroyed herself. This Babylon, which treated the Israelites mercilessly, would one day receive the same treatment at the hands of her own conquerors. Those who carried this out would be in a special position as they were the ones through whom these prophecies would be fulfilled. They would be happy because of the victory; and although God does not approve of the violence, His plan for executing justice upon the Babylonians for their treatment of Israel would be fulfilled.