Objection 1. It seems that God wills evils. For every good that exists, God wills. But it is a good that evil should exist. For Augustine says (Enchiridion 95): "Although evil in so far as it is evil is not a good, yet it is good that not only good things should exist, but also evil things." Therefore God wills evil things.
Objection 2. Further, Dionysius says (Div. Nom. iv, 23): "Evil would conduce to the perfection of everything," i.e. the universe. And Augustine says (Enchiridion 10,11): "Out of all things is built up the admirable beauty of the universe, wherein even that which is called evil, properly ordered and disposed, commends the good more evidently in that good is more pleasing and praiseworthy when contrasted with evil." But God wills all that appertains to the perfection and beauty of the universe, for this is what God desires above all things in His creatures. Therefore God wills evil.
Objection 3. Further, that evil should exist, and should not exist, are contradictory opposites. But God does not will that evil should not exist; otherwise, since various evils do exist, God's will would not always be fulfilled. Therefore God wills that evil should exist.
On the contrary, Augustine says (Qq. 83,3): "No wise man is the cause of another man becoming worse. Now God surpasses all men in wisdom. Much less therefore is God the cause of man becoming worse; and when He is said to be the cause of a thing, He is said to will it." Therefore it is not by God's will that man becomes worse. Now it is clear that every evil makes a thing worse. Therefore God wills not evil things.
I answer that, Since the ratio of good is the ratio of appetibility, as said before (5, 1), and since evil is opposed to good, it is impossible that any evil, as such, should be sought for by the appetite, either natural, or animal, or by the intellectual appetite which is the will. Nevertheless evil may be sought accidentally, so far as it accompanies a good, as appears in each of the appetites. For a natural agent intends not privation or corruption, but the form to which is annexed the privation of some other form, and the generation of one thing, which implies the corruption of another. Also when a lion kills a stag, his object is food, to obtain which the killing of the animal is only the means. Similarly the fornicator has merely pleasure for his object, and the deformity of sin is only an accompaniment. Now the evil that accompanies one good, is the privation of another good. Never therefore would evil be sought after, not even accidentally, unless the good that accompanies the evil were more desired than the good of which the evil is the privation. Now God wills no good more than He wills His own goodness; yet He wills one good more than another. Hence He in no way wills the evil of sin, which is the privation of right order towards the divine good. The evil of natural defect, or of punishment, He does will, by willing the good to which such evils are attached. Thus in willing justice He wills punishment; and in willing the preservation of the natural order, He wills some things to be naturally corrupted.
Reply to Objection 1. Some have said that although God does not will evil, yet He wills that evil should be or be done, because, although evil is not a good, yet it is good that evil should be or be done. This they said because things evil in themselves are ordered to some good end; and this order they thought was expressed in the words "that evil should be or be done." This, however, is not correct; since evil is not of itself ordered to good, but accidentally. For it is beside the intention of the sinner, that any good should follow from his sin; as it was beside the intention of tyrants that the patience of the martyrs should shine forth from all their persecutions. It cannot therefore be said that such an ordering to good is implied in the statement that it is a good thing that evil should be or be done, since nothing is judged of by that which appertains to it accidentally, but by that which belongs to it essentially.
Reply to Objection 2. Evil does not operate towards the perfection and beauty of the universe, except accidentally, as said above (ad 1). Therefore Dionysius in saying that "evil would conduce to the perfection of the universe," draws a conclusion by reduction to an absurdity.
Reply to Objection 3. The statements that evil exists, and that evil exists not, are opposed as contradictories; yet the statements that anyone wills evil to exist and that he wills it not to be, are not so opposed; since either is affirmative. God therefore neither wills evil to be done, nor wills it not to be done, but wills to permit evil to be done; and this is a good.
Source: Summa Theologica (New Advent)