By Rev. Jerry J. Pokorsky, The Catholic Thing, Dec. 29, 2019

The transition from the strict justice of the Mosaic Law to the mercy of “forgive thy enemies” in the teachings of Jesus sheds light on the glory of the Incarnation, which we are celebrating in a special way during these Christmas days, as well as on our participation in the life of Christ in the Sacrament of Penance.
King David viewed his adultery with Bathsheba and his murder of Uriah as offenses against God alone: “Against thee [God], thee only, have I sinned, and done that which is evil in thy sight, so that thou art justified in thy sentence and blameless in thy judgment.” (Ps. 51:4) The Psalmist also expected God to crush his enemies: “And in thy steadfast love cut off my enemies, and destroy all my adversaries, for I am thy servant.” (Ps. 143:12)
Desire for vengeance can seem as boundless as God’s mercy. It’s telling that when David provoked God with his census, he (again) feared human wrath more than divine punishment: “I am in great distress; let us fall into the hand of the Lord, for his mercy is great; but let me not fall into the hand of man.” (2 Sam. 24:14) ….
Read more here https://www.thecatholicthing.org/2019/12/29/the-god-of-justice-and-of-mercy/