Objection 1. It would seem that, properly speaking, evil is not the motive of mercy. For, as shown above (19, 1; I-II, 79, 1, ad 4; I, 48, 6), fault is an evil rather than punishment. Now fault provokes indignation rather than mercy. Therefore evil does not excite mercy.
Objection 2. Further, cruelty and harshness seem to excel other evils. Now the Philosopher says (Rhet. ii, 8) that "harshness does not call for pity but drives it away." Therefore evil, as such, is not the motive of mercy.
Objection 3. Further, signs of evils are not true evils. But signs of evils excite one to mercy, as the Philosopher states (Rhet. ii, 8). Therefore evil, properly speaking, is not an incentive to mercy.
On the contrary, Damascene says (De Fide Orth. ii, 2) that mercy is a kind of sorrow. Now evil is the motive of sorrow. Therefore it is the motive of mercy.
I answer that, As Augustine says (De Civ. Dei ix, 5), mercy is heartfelt sympathy for another's distress, impelling us to succor him if we can. For mercy takes its name "misericordia" from denoting a man's compassionate heart [miserum cor] for another's unhappiness. Now unhappiness is opposed to happiness: and it is essential to beatitude or happiness that one should obtain what one wishes; for, according to Augustine (De Trin. xiii, 5), "happy is he who has whatever he desires, and desires nothing amiss." Hence, on the other hand, it belongs to unhappiness that a man should suffer what he wishes not.
Now a man wishes a thing in three ways: first, by his natural appetite; thus all men naturally wish to be and to live: secondly, a man wishes a thing from deliberate choice: thirdly, a man wishes a thing, not in itself, but in its cause, thus, if a man wishes to eat what is bad for him, we say that, in a way, he wishes to be ill.
Accordingly the motive of "mercy," being something pertaining to "misery," is, in the first way, anything contrary to the will's natural appetite, namely corruptive or distressing evils, the contrary of which man desires naturally, wherefore the Philosopher says (Rhet. ii, 8) that "pity is sorrow for a visible evil, whether corruptive or distressing." Secondly, such like evils are yet more provocative of pity if they are contrary to deliberate choice, wherefore the Philosopher says (Rhet. ii, 8) that evil excites our pity "when it is the result of an accident, as when something turns out ill, whereas we hoped well of it." Thirdly, they cause yet greater pity, if they are entirely contrary to the will, as when evil befalls a man who has always striven to do well: wherefore the Philosopher says (Rhet. ii, 8) that "we pity most the distress of one who suffers undeservedly."
Reply to Objection 1. It is essential to fault that it be voluntary; and in this respect it deserves punishment rather than mercy. Since, however, fault may be, in a way, a punishment, through having something connected with it that is against the sinner's will, it may, in this respect, call for mercy. It is in this sense that we pity and commiserate sinners. Thus Gregory says in a homily (Hom. in Evang. xxxiv) that "true godliness is not disdainful but compassionate," and again it is written (Matthew 9:36) that Jesus "seeing the multitudes, had compassion on them: because they were distressed, and lying like sheep that have no shepherd."
Reply to Objection 2. Since pity is sympathy for another's distress, it is directed, properly speaking, towards another, and not to oneself, except figuratively, like justice, according as a man is considered to have various parts (Ethic. v, 11). Thus it is written (Sirach 30:24): "Have pity on thy own soul, pleasing God" [Cf. 106, 3, ad 1].
Accordingly just as, properly speaking, a man does not pity himself, but suffers in himself, as when we suffer cruel treatment in ourselves, so too, in the case of those who are so closely united to us, as to be part of ourselves, such as our children or our parents, we do not pity their distress, but suffer as for our own sores; in which sense the Philosopher says that "harshness drives pity away."
Reply to Objection 3. Just as pleasure results from hope and memory of good things, so does sorrow arise from the prospect or the recollection of evil things; though not so keenly as when they are present to the senses. Hence the signs of evil move us to pity, in so far as they represent as present, the evil that excites our pity.
Source: Summa Theologica (New Advent)