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.”Justice will be done. It may not be done now. If one of the things holding you back from letting it go is "It's wrong, justice hasn't been done, they're getting away with murder" then this promise is tailor-made for you.
(Romans 12:19 ESV)
God will lift from you the suicidal load of vengeance and carry it to one of two places: he will carry it to the cross if the person repents or he will carry it to hell where they will be forever. And you can't improve upon either of those. If they're in hell, you don't need add to their punishment. If their load was borne and forgiven and paid at the cross, you dishonor the Lord if you don't share in that forgiveness. It's a massive promise, "I will repay", and that is true for every single sin that has ever been committed anywhere in the universe at any time!