Objection 1. It would seem that cruelty is not opposed to clemency. For Seneca says (De Clementia ii, 4) that "those are said to be cruel who exceed in punishing," which is contrary to justice. Now clemency is reckoned a part, not of justice but of temperance. Therefore apparently cruelty is not opposed to clemency.
Objection 2. Further, it is written (Jeremiah 6:23): "They are cruel, and will have no mercy"; so that cruelty would seem opposed to mercy. Now mercy is not the same as clemency, as stated above (157, 4, ad 3). Therefore cruelty is not opposed to clemency.
Objection 3. Further, clemency is concerned with the infliction of punishment, as stated above (Question 157, Article 1): whereas cruelty applies to the withdrawal of beneficence, according to Proverbs 11:17, "But he that is cruel casteth off even his own kindred." Therefore cruelty is not opposed to clemency.
On the contrary, Seneca says (De Clementia ii, 4) that "the opposite of clemency is cruelty, which is nothing else but hardness of heart in exacting punishment."
I answer that, Cruelty apparently takes its name from "cruditas" [rawness]. Now just as things when cooked and prepared are wont to have an agreeable and sweet savor, so when raw they have a disagreeable and bitter taste. Now it has been stated above (157, 3, ad 1; 4, ad 3) that clemency denotes a certain smoothness or sweetness of soul, whereby one is inclined to mitigate punishment. Hence cruelty is directly opposed to clemency.
Reply to Objection 1. Just as it belongs to equity to mitigate punishment according to reason, while the sweetness of soul which inclines one to this belongs to clemency: so too, excess in punishing, as regards the external action, belongs to injustice; but as regards the hardness of heart, which makes one ready to increase punishment, belongs to cruelty.
Reply to Objection 2. Mercy and clemency concur in this, that both shun and recoil from another's unhappiness, but in different ways. For it belongs to mercy [Cf. 30, 1] to relieve another's unhappiness by a beneficent action, while it belongs to clemency to mitigate another's unhappiness by the cessation of punishment. And since cruelty denotes excess in exacting punishment, it is more directly opposed to clemency than to mercy; yet on account of the mutual likeness of these virtues, cruelty is sometimes taken for mercilessness.
Reply to Objection 3. Cruelty is there taken for mercilessness, which is lack of beneficence. We may also reply that withdrawal of beneficence is in itself a punishment.
Source: Summa Theologica (New Advent)