“But if through my lie God’s truth abounds to his glory, why am I still being condemned as a sinner? And why not do evil that good may come?—as some people slanderously charge us with saying. Their condemnation is just.” -Romans 3:7–8
While Paul might seem to be unnecessarily repeating his previous argument in verses 5-6, he is actually advancing his argument by answering a similar question a different way. It seems to me, these two closely-related arguments with different kinds of answers act as a rhetorical parallelism—like those seen in Hebrew poetry. On one hand, Paul is strengthening his point by presenting the same point twice. On the other hand, he is talking about two different (but closely related) ideas that can each provide its own answer.
In the first argument, he puts into bold relief the righteousness of God by comparing it to the (un)righteousness of men. In the second argument, he puts into bold relief the truthfulness of God by comparing it to the (un)truthfulness of men.
And, instead of an emphatic repudiation, he uses it as an opportunity to defend his true position against the slanderers (untruthful men) who have apparently made Paul notorious by saying he teaches that men should do evil that good may come. He will denounce this same accusation again in chapter six.
He concludes by saying their condemnation (God’s judgment against them) is just because it seems to be the case they are willingly speaking untruthfully about him. In any case, Just because God is able to make good come of men’s evil doesn’t mean he approves of evil. The providential work of a God does not make him complicit with men’s sin, but it does put in bold relief the majesty and power of God by comparing it to the character of men (slanderers).