I am definitely not an expert on this passage, but this is what I feel that Paul is saying... I will try to break it down for you as best I can.
Paul is basically going in circles here.
5 "But if our unrighteousness demonstrates the righteousness of God, what shall we say?
Paul is basically saying that our unrighteousness (us sinners), just makes God more righteous.
The God who inflicts wrath is not unrighteous, is He? (I am speaking in human terms.)
Then Paul wonders outloud, how anyone could call God unrighteous for taking vengeance (punishing) those people who breaks his law. Basically Paul is saying that our wretchedness is making God look good!
But then Paul starts getting confusing. In my mind, he is saying that if we are glorifying God, then why keep God's law?
We get 2 messages:
1. Keep the law
2. Don't keep the law, the law is good for those who can keep it, but no one can keep it, so why bother?
All in all, God's judging the world should take away the doubts about how God punishes/disciplines. It basically says that their (Jews at the time) condemnation is just, God deserves all the glory!
It's a hard excerpt. You have to kind of look at the whole chapter. Just don't get caught up in it Hope it helped a tiny bit.