giving vengeance to those who don't know God, and to those who don't obey the Gospel of our Lord Jesus,
Vengeance is not used in the sense of being spiteful, that is, it is not a "get-even" spirit as men often do, but it is from a Greek word indicating legal and judicial punishment on one who has shown disrespect for some law. Paul speaks of the Lord’s work of administering justice at the time He descends. The word "vengeance" is from the same root as the word "righteous" (verses 5 and 6). The idea is the administration of justice, not that of vindictive retaliation. The word from which vengeance comes is translated "vindicate" (Luke 18:7 RSV) and "punishment" (2 Corinthians 7:11 RSV).