Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, “Vengeance is mine, I will repay, says the Lord.” Romans 12:19
In this verse, Paul is quoting the Old Testament verse Deuteronomy 32:35. This is a very difficult verse for fallen people. God is telling us to trust Him for justice. The chapter goes on in Romans 12:20-21 to say, “To the contrary, “if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head.” “Do not be overcome by evil, but overcome evil with good.”
In his commentary, Matthew Henry says that “those that revenge are conquered, and those that forgive are conquerors.” Contention only escalates a situation. We are to be peacemakers. Meet cruel words and ill-meaning actions with calm kindness. Anger is fueled by anger, and is suffocated by persevering kindness. A kind response will bring you peace, and might change your adversary. If they continue to be adversarial, God is telling us that is not our concern. Turn your enemy over to God, and pray for him.