According to Psalm 137:9 we read that “How blessed will be the one who seizes and dashes your little ones against the rock.” The motive behind this line is important to consider.
Within this context the psalmist speaks about judgment brought on the Babylonian people who have taken the Jewish people into captivity. The psalmist also writes about the captors tormenting the people of God, a promise to remember Jerusalem, and a curse against the captors. It is most likely that the author is in exile and had likely witnessed heinous crimes and atrocities committed against his people, which would explain why he shows such disdain and anger at the Babylonians. In other words, he wishes violent ill fortune upon those who have harmed his fellow people.