Translated by Ralph Anzarouth and an anonymous friend
Cruelty is a great shortcoming and the antithesis of kindness and pity. In Aristo's opinion it will fall into one of five categories: the first is not to have pity on fellow creatures; the second is to refrain from helping or benefiting others according to their needs; the third refers to somebody who does not overlook offense; the forth is to punish others more than they deserve; the fifth involves harming one's fellow creatures without any cause or provocation.
The Torah says one should not be cruel even towards one's enemy, as it is said (Exodus 23, 5): "If you see your enemy's donkey crouching under its burden, you will help him to unload". It is also written (Proverbs 24, 17): "Do not rejoice in your enemy's downfall and do not let your heart exult etc.".
In this we clearly differenciate between the righteous and the evil person, as it is written (Proverbs 12, 10): "Even the pity of the evil is cruel", since even their compassion takes the form of cruelty, all the more so because their natural habits are malicious.
On the other hand, our Sages of blessed memory, in praising the children of Israel said (Talmud Bavli, tractate Yebamot 79a): "This Nation is distinguished by three attributes: they are compassionate, modest, and kind etc.". Biblical interpreters identified these by an acronym: gavar, as in the verse (Exodus 17, 11): "And Israel overcame1", since they are the antithesis of evil Amalek, whose cruelty against Israel was so strong that the Holy One blessed be He, cast off His trait of mercy2 and commanded us to wipe out the name of Amalek3. As it is written (ibid. 17, 16): "Hashem pledged on His throne to wage war against Amalek in every generation".
The trait of cruelty is found in the reptile called basilisk, a kind of lizard which kills even with its sight and has no trace of mercy in it, so that when it cannot find a living creature to poison it dries and burns the trees and foliage around it with its shrill whistling and foul exhalations.
The virulence of this trait is mostly directed against the weak and the humble among us. Therefore one should try to avoid it as much as possible. It is said in the book of Ben Sira: "Do not behave in your house like a merciless lion". The wise man said that when a servant is given a position of power he is more cruel than anybody else. Another wise man said: "Do not oppress the oppressed, since he will despair of his life". Cassiodoro said: "Nobody is more cruel than the person who wishes to get rich at the expense of the poor man's toil.
The ancients wrote about a woman called Syria who lusted after a man4. She pursued him and took her young brother with her whom she killed. She then cut him into pieces which she scattered here and there on the road. She did all this in order that when her father would chase after her he would find evidence of this extreme cruelty and would be too shocked to continue. In this way she would escape him. After she was united to the man she desired and had lived with him for some time, she had two sons by him. When her husband subsequently left her and fell in love with another woman, she murdered their two sons and drank their blood in order to annoy him. Then she wandered around the world and her fate was never to be known.
Notes of the translators:
 "Gavar" means "overcame", and it is also the acronym of the Hebrew words compassionate, modest, and kind ("Bayshanim, Rachmanim, Gomley Chasadim"). This seems to indicate that compassion, modesty and kindness are the means by which the Jewish people can defeat those who hate it.
 Nice quote from Amos 1, 11.
 To this end, Hashem commanded the Jewish People to erase all memory of Amalek.
 This is the famous tale of Medea and Jason.
The whole book Tzemach Tzadik in Hebrew written by Rabbi Leone di Modena (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 13 about cruelty is found at pages 27-28.