Just as death is not good but can be used for good, so the Law is not evil but can increase evil in us. That is, the Law (which acts as a restraint on sin--the "second" use of the law) is not evil, but "increases an illicit desire so long as the love of and joy in holiness is too weak to conquer the inclination to sin; and, without the aid of divine grace, it is impossible for man to love and delight in sanctity."
We all know this is true: what is our instinctive reaction when someone tells us not to do something? To do it. Or, to use the example I use in the classroom, how do we behave on the roads? We all know that the general purpose of highway safety laws is to encourage us to drive safely. And yet, knowing that we ought to drive safely, few of us really put effort into doing so. And when we make those highway safety laws more specific, as say with an explicit speed limit, our guilt in breaking the general purpose of the law (safe driving) is amplified as we break the specific law in question (the speed limit). "Thus, a sinner or a sin, by reason of the commandment, might become immeasurably worse."
What we see overall is that ones relationship to the Law and to death is defined not so much by either of those things as by the state of our hearts:
The Law is good because it is the prohibition of sin; death is evil because it is the penalty of sin. Wickedness makes a bad use not only of evil, but also of good. In the same way, holiness makes a good use not only of good, but also of evil. Thus, sinners make a bad use of the Law, although the Law is good, while saints make a good use of death, although death is an evil.Chapter 6:
Physical death--the separation of the soul from the body--"is not good for anyone." And yet, however terrible this process may be (and it is terrible--we must not romanticize the punishment for sin!), "if it is suffered with faith and piety, it increases the merit of patience... Although death is the punishment of sin, sometimes it secures for the soul a grace that is a security against all punishment for sin."
Again, the evil of death can be turned to the glory of God and the good of those who believe in Him.
Here, we have a partial correction to the previous problematic view of baptism in chapter 4: "those unbaptized persons who die confession the name of Christ... receive the forgiveness of their sins as completely as if they had been cleansed by the waters of baptism." Here we see that Augustine clearly understands that salvation has to do with the orientation of the heart, not the outward symbol of the Gospel (again, not to slight that symbol!). Even more, we know that such men who confess Christ are not "unbaptized" in the sense that they are unsaved--they are 'baptized' by the Spirit in their regeneration. That they have not been baptized by the hand of man has no bearing on their salvation.
Now, that said, denying Christ in the face of persecution is not necessarily the unpardonable sin. "If through fear of death they did deny Christ, even that sin could be canceled by the baptism which washed away even the heinous outrage of those who killed Christ." It is better to hold to Christ even in the face of persecution, but giving in to weakness and temptation is not a sign that one has not been regenerated by the baptism of the Spirit. The atonement is sufficient to cover even this sin, though we should not give into the sin but should instead understand that death has been defeated and that as believers through Christ the evil of death is not "something to be feared," but rather is "something to be endured."
After all, why should a Christian fear physical death when we will never face the true, second death? "His motive in facing a partial death is to escape total death and, above all, a death which is eternal." "Death," Augustine writes, "is good for no one, but it may become meritorious if suffered to retain or to gain some good."
When we look to a higher good than the good of continuing in this life--namely, the eternal life found in the presence of God--we find that death loses its sting.
It can be hard to pinpoint the moment of death, and even harder still to define "dying," since we're all moving towards the (temporary) separation of soul and body. It's perhaps just easiest to use conventional language and say that we recognize "dying" when we see it, usually by recognizing the pain that it brings on.