We know that the law is good if one uses it properly. We also know that law is made not for the righteous but for lawbreakers and rebels, the ungodly and sinful, the unholy and irreligious; for those who kill their fathers or mothers, for murderers, for adulterers and perverts, for slave traders and liars and perjurers—and for whatever else is contrary to the sound doctrine that conforms to the glorious gospel of the blessed God, which he entrusted to me.
It seems that Paul is both expanding on and implying what he said in Romans 7:7, “What shall we say, then? Is the law sin? Certainly not! Indeed I would not have known what sin was except through the law.” Because the false teachers sought to teach the Law as a regulation for the Christian life, Paul shows just how the regulatory function of the Law looks when it is applied to sinful humanity. When the Law is applied to life as a regulation, it results only in condemnation of sin. That condemnation is not for the justified (Rom 8:1) but for lawless rejecters of Paul’s gospel. The Law, when used properly, is on Paul’s side against those who wish to be teachers of the Law. That’s why it’s so obvious to Paul that these teachers “don’t know what they are talking about” (v. 7).
However, those who submit to Paul’s gospel will not evaluated by the law. They will be granted mercy as Paul, the chief among sinners, was granted mercy (v. 16).
Theologically, what is the role of the Law for today?
I think using the Ten Commandments to show unbelievers their personal sin is not necessarily a bad idea. Paul evidently intended the list here to roughly parallel those Sinaitic injunctions: the fifth through the ninth commandments are especially clear and in order (though the fourth commandment is conspicuously absent). However, we must make clear to unbelievers that keeping the law “from now on” is both impossible and ineffective in erasing past sins. In addition one aspect of genuine conversion and union with Christ is dying to the law (Rom 7:4). The law’s condemnation is laid to rest along with its regulatory authority over believer's lives. The Law is good, but it works for Paul’s gospel, not against it.