What role does the Law play in relation to the old sin nature?
The Law exercises the old nature, making it operative as a lawbreaker. Without the Law, the old nature was dead and inoperative; when the commandment came, old Adam woke up, recognized coveting, and died. This death of old Adam precedes new life and salvation.
Les explains from verse 9 that without the Law the old nature was dead and inoperative, but it was the Law that exercised it. When the commandment came, "Thou shalt not covet," old Adam woke up, and old Adam in Saul's make-up said, "Hey, that's me, I'm a coveter." Then he died. Les says he had to die in the realm of that old Adam who was a Lawbreaker, and with the death of old Adam comes new life—that is salvation. When our old Adam is put to death because he was a Lawbreaker, and we become a believer in the finished work of the Christ, which is the Gospel, we are a new creation, alive, with eternal life. Paul constantly refers to that, and Peter said in all his epistles this is all he talks about.
For I was alive without the law once: but when the commandment came, sin revived, and I died.