When you harbor feelings of vengeance you show that you don’t have faith in what God has promised. Vengeance belongs to the Lord. He will repay. https://www.desiringgod.org/messages/the-word-of-god-is-at-work-in-you/ https://www.desiringgod.org/messages/the-word-of-god-is-at-work-in-you/excerpts/how-to-battle-bitterness