Paul addressed some questions regarding the big mistakes that we have done, like the role of circumcision and the effect on God's faithfulness. Another question that Paul addresses is regarding God's wrath (which as we see is his passion to stop evil).
In particular, the question phrasing is whether or not doing wrath is unjust "if our unrighteousness demonstrates the righteousness of God" (3:5). And the answer is no! There are two reasons why God's wrath/passion is not unjust.
The first reason is the importance of God judging the world (3:6). If you're familiar with history or even current events, you're probably familiar with the many, many horrible actions done throughout human history. You also probably don't like inhumane mistreatment, especially when others do it to you. God does not like sin because sin ruins lives, and God values human life. Many evil stems from an impure heart, and impure hearts lasting forever is not good due to the chance to do evil (not to mention God planned a place intentionally to be sin-free).
The second reason is the possible excuse to do evil for good to come (3:8). Aside from the importance of dealing with evil, there's also the phrasing that can be considered "slanderous". Some people even thought Paul's comments could be interpreted that way, though the idea is rebutted by how God's judgment plan. Also, throughout history, many people do not like the idea of being rewarded for doing evil.
All in all, as Paul wrote, "[the] condemnation is just". In other words, God correctly concludes that we are guilty for at least one bad thing that we have done. But God provided a way for us to be cleared, and Paul will go into more detail later in the chapter.