Charles H. Spurgeon on Psalms 137:9

Happy shall he be that taketh, etc. That is, so oppressive hast thou been to all under thy domination, as to become universally hated and detested; so that those who may have the last hand in thy destruction, and the total extermination of thy inhabitants, shall be reputed "happy" —shall be celebrated and extolled as those who have rid the world of a curse so grievous. These prophetic declarations contain no excitement to any person or persons to commit acts of cruelty and barbarity; but are simply declarative of what would take place in the order of the retributive providence and justice of God, and the general opinion that should in consequence be expressed on the subject; therefore praying for the destruction of our enemies is totally out of the question. —Adam Clarke.