Objection 1. It would seem that pain has more of evil than fault. For fault is to pain what merit is to reward. But reward has more good than merit, as its end. Therefore pain has more evil in it than fault has.
Objection 2. Further, that is the greater evil which is opposed to the greater good. But pain, as was said above (Article 5), is opposed to the good of the agent, while fault is opposed to the good of the action. Therefore, since the agent is better than the action, it seems that pain is worse than fault.
Objection 3. Further, the privation of the end is a pain consisting in forfeiting the vision of God; whereas the evil of fault is privation of the order to the end. Therefore pain is a greater evil than fault.
On the contrary, A wise workman chooses a less evil in order to prevent a greater, as the surgeon cuts off a limb to save the whole body. But divine wisdom inflicts pain to prevent fault. Therefore fault is a greater evil than pain.
I answer that, Fault has the nature of evil more than pain has; not only more than pain of sense, consisting in the privation of corporeal goods, which kind of pain appeals to most men; but also more than any kind of pain, thus taking pain in its most general meaning, so as to include privation of grace or glory.
There is a twofold reason for this. The first is that one becomes evil by the evil of fault, but not by the evil of pain, as Dionysius says (Div. Nom. iv): "To be punished is not an evil; but it is an evil to be made worthy of punishment." And this because, since good absolutely considered consists in act, and not in potentiality, and the ultimate act is operation, or the use of something possessed, it follows that the absolute good of man consists in good operation, or the good use of something possessed. Now we use all things by the act of the will. Hence from a good will, which makes a man use well what he has, man is called good, and from a bad will he is called bad. For a man who has a bad will can use ill even the good he has, as when a grammarian of his own will speaks incorrectly. Therefore, because the fault itself consists in the disordered act of the will, and the pain consists in the privation of something used by the will, fault has more of evil in it than pain has.
The second reason can be taken from the fact that God is the author of the evil of pain, but not of the evil of fault. And this is because the evil of pain takes away the creature's good, which may be either something created, as sight, destroyed by blindness, or something uncreated, as by being deprived of the vision of God, the creature forfeits its uncreated good. But the evil of fault is properly opposed to uncreated good; for it is opposed to the fulfilment of the divine will, and to divine love, whereby the divine good is loved for itself, and not only as shared by the creature. Therefore it is plain that fault has more evil in it than pain has.
Reply to Objection 1. Although fault results in pain, as merit in reward, yet fault is not intended on account of the pain, as merit is for the reward; but rather, on the contrary, pain is brought about so that the fault may be avoided, and thus fault is worse than pain.
Reply to Objection 2. The order of action which is destroyed by fault is the more perfect good of the agent, since it is the second perfection, than the good taken away by pain, which is the first perfection.
Reply to Objection 3. Pain and fault are not to be compared as end and order to the end; because one may be deprived of both of these in some way, both by fault and by pain; by pain, accordingly as a man is removed from the end and from the order to the end; by fault, inasmuch as this privation belongs to the action which is not ordered to its due end.
Source: Summa Theologica (New Advent)