Read 1 Samuel 24:17-18
Then [Saul] said to David: "You are more righteous than I; for you have rewarded me with good, whereas I have rewarded you with evil. And you have shown this day how you have dealt well with me; for when the Lord delivered me into your hand, you did not kill me."
A gentleman who had held many important positions in public life went to a friend in great anger over a real injury he had received from a prominent politician. He was considering how to react resentfully in the most effective manner. After relating the particulars to his friend, he asked if it would be manly to resent it. "Yes," replied his friend, "it would doubtless be manly to resent it, but it would be godlike to forget it."
David chose to let God be his example. When Saul entered a cave to attend to his needs, he didn't know that David and his 400 men were hiding in the recesses of that cavern. David had him at a severe disadvantage and his men urged him to seize the opportunity to take revenge on his enemy. But David refused. Rather than seeking to repay Saul for the evil he had done him, he secretly cut off a piece of Saul's robe (as proof of what he could have done) and allowed the king to leave without knowing how close he had been to death. Only later did Saul realize the mercy David had shown.
God's way is to show mercy rather than extract vengeance. Romans 5:8 says, "But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us." Even while we were God's enemies, He had compassion for us and provided a way of salvation.
Are you thinking about getting even with someone? Don't do it. Try God's way instead. Return good for evil. Someday you'll be glad you did.
Evil for evil is man's way; good for evil is God's way.