Saint Thomas Aquinas: “by the name of evil is signified the absence of good.”
Summa Theologica, I, Q. 48, A. 1.
There are three types of evil:
- moral evil
Sin is moral evil, and moral evil is sin.
- physical evil
Something of a misnomer, since physical evil includes deprivations that are not physical.
- metaphysical evil
Theologians disagree on how this should be defined, but it is perhaps best and most simply represented as the lack of goodness in any finite created things – finiteness, as compared to the infinite goodness of God.
None of the acts of God are moral evil, since morality is defined by the goodness of the Nature of God, and God cannot deny Himself.
God does sometimes will physical evil, such as the sufferings of Purgatory, or such as the destruction of a city as a punishment for sin.
Sacred Scripture says this: “Shall there be evil in a city, which the Lord hath not done?” (Amos 3:6, Challoner version). The note for this verse says: “Evil in a city. He speaks of the evil of punishments of war, famine, pestilence, desolation, &c., but not of the evil of sin, of which God is not the author.”