Romans 3:5-8
5 But if our unrighteousness commend the righteousness of God, what shall we say? Is God unrighteous who taketh vengeance? (I speak as a man)
6 God forbid: for then how shall God judge the world?
7 For if the truth of God hath more abounded through my lie unto his glory; why yet am I also judged as a sinner?
8 And not rather, (as we be slanderously reported, and as some affirm that we say,) Let us do evil, that good may come? whose damnation is just.
Abridged Commentary - Romans 3:8
3:8 The word translated "damnation" in verse 8 in the KJV, "krima" (κρίμα), simply refers to "judgment", or "condemnation". Eternal condemnation is not in view here. The NKJV has a better rendering, "Their condemnation is just". This refers to the just condemnation of those who would say that, in some way, sinning exalts God, in this context, by ostensibly highlighting His holiness. The Apostle Paul deals with a similar hypothetical objection in Romans 6:1-2.
The Grace New Testament Commentary - Romans 3:7-8
3:7-8 The argument Paul is refuting is that if God's truth is magnified by my untruth (i.e., my lie in denying His revealed truth), then why would my lie constitute me as a sinner in the judgment? Paul is still thinking here of the unbelief referred to in v 2, so that when the Jew expressed a belief contrary to his own Scriptures, that false belief would be a lie. The basic thought seems to be that God's truth is enhanced by my untruth to His glory, and Paul is not concerned with the details of how that occurs.
If one took the approach Paul is referring to, he would be denying God's right to judge a person as a sinner if the sinful actions had somehow redounded to God's glory. But this premise could lead to yet a further (ludicrous) distortion, in which one might say, "Let us do wicked things so that good things may happen." Paul himself had been charged by some as teaching this very thing. This claim he regards as slanderous.
This whole line of thought is obviously wrong on its face, and Paul simply states that those who make such claims deserve the judgment passed on them. In Paul's view, such people are already under sentence of judgment because of the manifest untruth of such claims. God's right to "judge the world" (v 6) cannot be denied. To Paul, the whole argument is not only flawed, but perverse.