When we talk of good and evil wills, we are not of course speaking of plants, animals, and the "natural" world, which lives and dies in accord with "the harmony of the universe." That there is change and decay in nature is no bad thing per se. In fact, it is simply it's place in the scheme of creation.
If the beauty of this order fails to delight us, it is because we ourselves, by reason of our mortality, are so enmeshed in this corner of the cosmos that we fail to perceive the beauty of a total pattern in which the particular parts, which seem ugly to us, blend in so harmonious and beautiful a way.I would suggest that Augustine here is perhaps understating the effects of sin--there is ugliness in the death and decay of the animal world, though it is an ugliness that flows out of our rebellion, not theirs.
That said, Augustine's response is exactly the right one:
That is why, in those situations where it is beyond our power to understand the providence of God, we are rightly commanded to make an act of faith rather than allow the rashness of human vanity to criticize even a minute detail in the masterpiece of our Creator.It is not our place to blame God for the violence of the natural world. Yet, even with this rough natural reality, we see a display of God's glory in His created handiwork. The ugliness of the plague of frogs by no means lessens the beauty of their place in the ecosystem. That fire can destroy, hurt, and kill does not detract from its beauty or usefulness. Our problem when we make such judgments is one of perception. When we look as humans with small and selfish eyes, we see a world that does not meet our standards. But when we look at God, we see the world recast as it really is under His Divine sovereignty and as it exists by His Holy will.
Thus, all of creation is good simply because a good God made it--everything that is has "its own measure of being, its own beauty, even, in a way, its own peace." What's more, when each does what it is assigned to do in the created order "it best preserves the full measure of being that was given to it." When plants and animals die, they are promoting the good of those beings that will live forever as they push history and life along its ordained path to its culmination in the City of God. They "follow the direction of Divine Providence and tend toward the particular end which forms a part of the general plan for governing the universe."
The conclusion from all this is that God is never to be blamed for any defects that offend us, but should ever be praised for all the perfection we see in the natures He has made.If we do not see such perfection, that is our sinful eyes and not a defect in Him or in His good work in creation.
Augustine in this chapter takes on one of the most difficult and subtle problems in all of philosophy--the problem of the origin of evil in the will. (If you want to read the definitive work on this, Jonathan Edwards' Freedom of the Will is the book to grapple with.)
Augustine begins by pointing out that the angels fell when they choose a lesser, dependent, and relative being (themselves) over "Him whose Being is absolute." This is the very definition of pride: elevating the unworthy over the worthy.
But how did they do this? It was an act of their wills, to be sure, but what was the cause of that act? "If one seeks for the efficient cause of their evil will, none is to be found. For, what can make the will bad when it is the will itself which makes an action bad?" We judge something as "evil" when it is done with a bad will driving it. If I electrocute someone with the intent of damaging them because I enjoy hurting people, we understand that is an evil action. If I electrocute someone with the intent of restarting their heart, we understand that is a good action. The virtue of the action is determined by the will that drives it.
But, if we ask ourselves "what caused that first evil willing?" If we assume that there was a prior act of evil willing, we quickly get lost in an infinite regression because we then have to ask "what caused the cause of the first evil willing?" Instead, Augustine has to conclude that there could not have been a "will" behind the first evil willing. (He also dismisses the possibility that a good will caused the first evil willing--"one would have to be foolish to conclude that a good will makes a bad will.")
But! We also cannot assume that the evil will has always existed: "If I am told that nothing made the will evil but that it was always so, then I ask whether or not it existed in some nature."
If the will was not tied to some nature, then it did not exist at all--there is no abstract, empty will floating around out there somewhere in the ether. The will is tied to existing beings and requires a nature to exist.
Likewise, the first evil will must have come from a good nature, since in order to be "evil" it must have done some damage: "it vitiated, corrupted, injured that nature, and, therefore, deprived it of some good." If the nature was evil to begin with, then an evil will would do it no damage, and as a result we couldn't really call it "evil." So either way, the evil will could not have existed from eternity past.
Therefore, Augustine concludes, "the only remaining suggestion is that the cause of the evil will was something which had no will." But this "something" must, by the rules of logic, be either superior, inferior, or equal to the will. If it is "superior, then it was better", and therefore must be good and could not force the will to turn to evil. The same is true if it is equal. Therefore, whatever this something is that corrupts the will must be some inferior thing.
Now, as we've already established repeatedly, everything that was created was created good--including this inferior thing that corrupts the will. Which raises the question: "How, then, can a good thing be the efficient cause of an evil will? How, I ask, can good be the cause of evil?"
Augustine's answer to these questions has become the definitive (if somewhat unsatisfying--to me, at any rate) Christian answer to the problem of evil:
For, when the will, abandoning what is above it, turns itself to something lower, it becomes evil because the very turning itself and not the thing to which it turns is evil. Therefore, an inferior being does not make the will evil but the will itself, because it is a created will, wickedly and inordinately seeks the inferior being.That is, evil enters not in a thing that exists, but in an action that we take--an inclination of souls and mind that chooses a lesser good (still a good thing, since all of creation is good!) over God. And so in a sense, when we choose a lesser good--a part of creation rather than the Creator--we are ultimately choosing not to obey the mandate of our nature, but are rather embracing the nothingness out of which we are made and filling our souls with a vacuum bereft of goodness.
Again, the book to read here is Jonathan Edwards' Freedom of the Will, in which he takes Augustine's idea and point by Scripturally relentless point drives home its implications for human anthropology. And if you've made it this far, here's your reward for patience/punishment for stubbornness: my short(-ish) summary of Edwards' work.