We really don’t like this instruction. It’s importance is in the repetition. Solomon is restating the words of God to Moses.
(Deu 32:35 KJV) To me belongeth vengeance, and recompence; their foot shall slide in due time: for the day of their calamity is at hand, and the things that shall come upon them make haste.The thought is repeated by Paul in Romans and by the writer to the Hebrews.
We are not to take vengeance. Jesus puts it in a positive way when He tells us to turn the other cheek, go the extra mile and so forth in the Sermon on the Mount of Matthew 5.
So? Don’t even think about it. Forgiveness is key to our salvation both getting and giving.
(Mat 6:15 KJV) But if ye forgive not men their trespasses, neither will your Father forgive your trespasses.