There are passages in the Bible that are difficult to defend. Psalm 137:9 is one of those verses. The text reads, "How blessed will be the one who seizes and dashed your little ones against the rock." What a horrible picture of the cruelties of war!
The following considerations are simply presented as 'food for thought': (1) if the Bible were simply a work of man, why would one ever include this text?! (2) the verse doesn't say 'one SHOULD behave' this way rather that there is coming a predicted day of reckoning for Babylon, (3) it's pretty likely that some will one day look back and also question the writer's motives in songs like "Straight Outta Compton" by NWA and other lyrics which express deep emotion and anger which are being produced in our generation, (4) the previous verse (v. 8) communicates a desire for repayment and recompense. War is ugly, pain-filled and brutal.
Today, the motive that should move us toward our enemies is expressed in Romans 12:17–19, “Do not repay anyone evil for evil...do what is right in the eyes of everyone...live at peace with everyone. Do not take revenge...leave room for God’s wrath, for it is written: ‘It is mine to avenge; I will repay,’ says the Lord.”
While Scripture contains hard things to understand, my limited intellect and flawed understanding does not affect the truth or perfect reasoning contained in God's Word. Jesus life, death and resurrection is all the proof needed to trust His revelation!