Objection 2. Further, "being" and "thing" are convertible. If therefore evil is a being in things, it follows that evil is a thing, which is contrary to what has been said.St. Thomas replies that since the perfection of the universe requires inequality in things, and since differences of goodness imply privations of one sort or another, there are necessarily evils of various sorts in creation.
Objection 3. Further, "the white unmixed with black is the most white," as the Philosopher says (Topic. iii, 4). Therefore also the good unmixed with evil is the greater good. But God makes always what is best, much more than nature does. Therefore in things made by God there is no evil (ST I Q48 A2).
As, therefore, the perfection of the universe requires that there should be not only beings incorruptible, but also corruptible beings; so the perfection of the universe requires that there should be some which can fail in goodness, and thence it follows that sometimes they do fail. Now it is in this that evil consists, namely, in the fact that a thing fails in goodness. Hence it is clear that evil is found in things, as corruption also is found; for corruption is itself an evil (ibid.; emphasis added).An incorruptible being in Aquinas' reckoning is one that does not change, because corruption is a species of change: hence, by incorruptible beings, he means angels.
St. Thomas' reply to Objection 3 above is worth including here:
God and nature and any other agent make what is best in the whole, but not what is best in every single part, except in order to the whole, as was said above. And the whole itself, which is the universe of creatures, is all the better and more perfect if some things in it can fail in goodness, and do sometimes fail, God not preventing this. This happens, firstly, because "it belongs to Providence not to destroy, but to save nature," as Dionysius says (Div. Nom. iv); but it belongs to nature that what may fail should sometimes fail; secondly, because, as Augustine says (Enchir. 11), "God is so powerful that He can even make good out of evil." Hence many good things would be taken away if God permitted no evil to exist; for fire would not be generated if air was not corrupted, nor would the life of a lion be preserved unless the ass were killed. Neither would avenging justice nor the patience of a sufferer be praised if there were no injustice (ibid., ad 3).There is no getting around the fact that in some sense there is "evil" in the world as created by God. The perfection of the whole is what must be in view when God says in Genesis that the creation was "very good", since there are different degrees of perfection among creatures.
I think what St. Thomas says about God not "destroying" but "saving" nature is very powerful. We are not puppets on a string, which would be contrary to our nature as God's image-bearers and moral agents. It's this sort of thing that highlights the figurative language of Isaiah 65:25:
The wolf and the lamb shall feed together; the lion and the ox shall eat straw; and dust shall be the serpent's food: they shall not hurt nor kill in all my holy mountain, saith the Lord.It's contrary to a lion's nature to eat straw; Isaiah does not mean to say that carnivores will become herbivores, but rather he is saying something about the radically different sort of peace and harmony that will characterize the new heavens and the new earth (v. 1). As the editors of the CCD Bible say about Isaiah 11:6-9:
This picture of the idyllic harmony of paradise is a dramatic symbol of the universal piece and justice of Messianic times.Until then - until there is a new heavens and a new earth - we find imperfection in this world, and corruption that is a feature of how God made the world. That's why we have maggots and worms and buzzards: to clean up the corruption. The world works, and it is very good.