What does the law show about the old Adam's true nature?
The law reveals the old Adam as rebellious and evil, bent to ignore the law. Though the law itself is good and perfect, it exposes sin, convicts the sinner, and works death, causing sin to become exceedingly sinful, so that old Adam is seen for what he really is.
Les explains that the law was not the problem; it was good and perfect. But it was killing and convicting the old Adam, so that sin, old Adam, by the commandment might become exceedingly sinful. Les connects this to Romans 3:20, where by the deeds of the law no flesh is justified, for by the law is the knowledge of sin. The law thus exposes old Adam's rebellion, evil bent, and refusal to heed the law, showing him for what he really is.