Paul begins by talking about the coming "Judgment Day". A day that brings relief to the afflicted (in this case the Thessalonian church which is enduring persecution) and vengeance on evil doers and unbelievers. Paul's language has a hard edge to it but the hardest of all is verse 9 " . . . separated from the presence of the Lord and from the glory of his might. . ." I cannot imagine a worse fate that to be separated from the source and Lord of all life. Yet God made a risky universe where great things are possible. It is a risky universe where perfection lies in the fulfillment of our purpose. We were created to be the object of God's love and we never can experience completeness or wholeness or holiness outside of that love. But this risky universe gives us the power to choose who or what we will follow and how we will live our lives. We can choose to live surrendered to God's love and grace (and thus be complete) or we can choose to rule our own universe -- a somewhat pathetic choice, but nonetheless a common one. There can be no love without choice and with choice comes the option to live outside of God's love. Whatever images the bible chooses to use for the punishment of the wicked and evil doers . . . none could be worse than this: facing eternal separation from God. That, by definition would be annihilation.
Once again Paul emphasizes that when Christ returns that is the end of the story as we have lived it and a new story is about to begin.