Great message (and short video) from John Piper on bitterness. When I harbor feelings of vengeance, I show that I do not have faith in what God has promised.
Vengeance belongs to the Lord. He will repay.
If it is possible, as far as it depends on you, live at peace with everyone. Do not take revenge, my dear friends, but leave room for God’s wrath, for it is written: “It is mine to avenge; I will repay,” says the Lord.
Romans 12:18-20 (NASB)