Bitterness is a destructive root of many kinds of sin (Heb. 10:25). God makes us this promise as we think about Defeating Bitterness…”Vengeance is mine. I will repay. (Rom 12:18).”
This promise says, “Yes, an outrage has been committed against you. Yes, it deserves to be severely punished. Yes, the person has not yet experienced that punishment. But, NO, you cannot be the one to punish, and you may not go on relishing personal retribution. Why? Because God will see to it that justice is done. God will repay. You cannot improve on his justice. He sees every angle of the evil done against you—far better than you can see it. His justice will be more thorough than any justice you could administer.” If you hold a grudge, you doubt the Judge (from John Piper, in “Battling Unbelief”).