In Psalm 137 verses 8-9, a very brutal statement is made. It says
8 O daughter of Babylon, who are to be destroyed,
Happy the one who repays you as you have served us!
9 Happy the one who takes and dashes
Your little ones against the rock!
One wonders at such words, being inspired by a God who is loving and kind.
We must remember that in the Old Testament what took place in the natural and physical realm, primarily did so as representing spiritual truths. The Babylonians were the enemy of Israel taking them into captivity, robbing them of their freedom and the blessing of God. They were a destructive force and influence in their lives.
The real enemy of mankind is not physical, or in the natural realm but is spiritual and may be summarised by the word sin. Sin just like the Babylonians captures us, robs us of our freedom and the blessing of God. It is destructive in our life and the lives of those around us, therefore God tells us not to have any tolerance or mercy towards it. We are to be brutal against it, as in the sentiment expressed in these two verses.
A New Testament commentary on these two verse would be Romans 8:13:
13 For if you live according to the flesh you will die; but if by the Spirit you put to death the deeds of the body, you will live.
We are by the Spirit to put to death the deeds of the body. The deeds of the body are sin in our life, so we are to put to death, be brutal against sin, because it is so destructive to us and those around us.
We are to be brutal to the Babylonians!