Gabe.
Your trying to compare God divinely chastising a saved man( a believer) in the church for being led by his flesh ( 1 Cor 5:5) to a eternally( forever, without end) ruined unbeliever( a un-saved man) who will be separated from the Lord in hell and the glory of His power because of his unbelief. (2 Thess 1:9) Not the same. God is not chastising the unbeliever for corrective purposes( you must be a believer for discipline, Hebrews 12:6-8), but dealing out retribution for not believing the gospel.
The sin nature or sin is not the subject of the context in 2 Thess chapter one. I don’t know how you come up with that. You have already admitted that your interpretration to be in error to the context of the subject in this chapter.
Every scripture must be left in its setting in context to the subject the chapter is talking about. It is never to be lifted out of its setting to be interpreted. Context is everything, Gabe. When you compromise context it leads to error.