Paul's message of grace was hard to swallow for the Jew of this time and even for many Christians today. His revelation on grace and faith was unrivaled among his peers. He constantly had to defend the gospel of grace to nay-sayers and critics. This passage of scripture is no exception.
On the one hand, Paul is saying that the argument that our sin glorifies God's mercy, is completely carnal and self-seeking. Basically seeking permission to engage in their desires while imputing God's character as an unfair judge. Paul maintains that God alone can judge and is completely reasonable and just in doing so. Therefore, argument against being condemned as a sinner is full of holes. While, it is true that our sin does illuminate how gracious God is to us, it still doesn't excuse the sin. God never wanted us to know or experience sin, so we know that our sin is never a good thing.
On the other hand, Paul had to defend himself against critics who slandered him constantly. Paul's revelation of grace was so dependant on what God had done, that it cause many people to accuse him of encouraging people to sin. His response... 'Their condemnation is deserved." Meaning that these accusations were completely false and utterly unfounded.
Today, I pray that you realize that God's mercy is not permission to go and live a life of sin. Romans 2:4 says, that it is God's kindness leads to repentance. Repentance is simply to turn and go the other way. So instead of continuing on in the way of sin, I hope you recognize God's abundant mercy and grace poured out upon you through His son Jesus. And for that knowledge to propel you into living a life worthy of your calling.