Vv. 6-8 For it is indeed just of God to repay with affliction those who afflict you, and to give relief to the afflicted as well as to us, when the Lord Jesus is revealed from heaven with his mighty angels in flaming fire, inflicting vengeance on those who do not know God and on those who do not obey the gospel of our Lord Jesus.
Personally, I could live without Second Thessalonians in the Canon of Scripture. I don't think it offers much positive and there is plenty negative, like the verses quoted above. Scholars have long debated whether or not Paul really wrote this letter, in part because the verses above sound like Paul on steroids.
I have trouble with notions of "the vengeance of God." I think it is making God in our image. Vengeance is our thing, not God's. I believe if nothing else the cross proved that. Are there things that make God angry? Of course. I certainly hope so. If God is not angry at the extent of poverty in a city such as ours, for instance, than I do not really want to have anything to do with that God. But God has taught us through the vulnerability of the cross how to turn our anger into passion--the passion of transformative love.
Do I hope there will come a day when my enemies will get what they deserve? Most days, yes. But "the angel of my better nature" tells me that in Christ Jesus what they "deserve" is a chance at being reconciled to God, and therefore to all they have harmed, even at the end (I'll get the same chance, and so will you). I've known a few of them who are likely to say "no thanks." That will be their choice. Do they then get the vengeance and the fire? I think what they get is the death they already have and seem to want to keep having.
Perhaps I can be grateful that there are passages like this that help me to confront my own need for vengeance and vindication, and to remind me that my vindication was won long, long ago outside the walls of Jerusalem, and because of the act of the man Jesus on the cross, I need not seek vengeance ever again.