“Cruelty is cruelty under whatever colouring it may appear; and whether exercised on a man or on a fly, cruelty is still cruelty. It matters not whether the victim be furnished with two legs or with four, with wings, with fins, or with arms; where there is sensation, there is subject for cruelty, and in proportion to the degree of sensation will its action operate.”

In an interview with Carol Rittner and Sandra Meyers in Courage To Care - Rescuers of Jews during the Holocaust, NYU Press, 1986, p. 2. Also quoted by Yad Vashem http://www.yadvashem.org/righteous/about-the-program.html and Nicholas Kristoff in The Silence of the Bystanders https://www.nytimes.com/2006/03/19/opinion/the-silence-of-bystanders.html, New York Times (March 19, 2006).
Source: Night

Preface, Leading Case of Jesus Christ
1930s, On the Rocks (1933)
Context: I dislike cruelty, even cruelty to other people, and should therefore like to see all cruel people exterminated. But I should recoil with horror from a proposal to punish them. Let me illustrate my attitude by a very famous, indeed far too famous, example of the popular conception of criminal law as a means of delivering up victims to the normal popular lust for cruelty which has been mortified by the restraint imposed on it by civilization. Take the case of the extermination of Jesus Christ. No doubt there was a strong case for it. Jesus was from the point of view of the High Priest a heretic and an impostor. From the point of view of the merchants he was a rioter and a Communist. From the Roman Imperialist point of view he was a traitor. From the commonsense point of view he was a dangerous madman. From the snobbish point of view, always a very influential one, he was a penniless vagrant. From the police point of view he was an obstructor of thoroughfares, a beggar, an associate of prostitutes, an apologist of sinners, and a disparager of judges; and his daily companions were tramps whom he had seduced into vagabondage from their regular trades. From the point of view of the pious he was a Sabbath breaker, a denier of the efficacy of circumcision and the advocate of a strange rite of baptism, a gluttonous man and a winebibber. He was abhorrent to the medical profession as an unqualified practitioner who healed people by quackery and charged nothing for the treatment. He was not anti-Christ: nobody had heard of such a power of darkness then; but he was startlingly anti-Moses. He was against the priests, against the judiciary, against the military, against the city (he declared that it was impossible for a rich man to enter the kingdom of heaven), against all the interests, classes, principalities and powers, inviting everybody to abandon all these and follow him. By every argument, legal, political, religious, customary, and polite, he was the most complete enemy of the society of his time ever brought to the bar. He was guilty on every count of the indictment, and on many more that his accusers had not the wit to frame. If he was innocent then the whole world was guilty. To acquit him was to throw over civilization and all its institutions. History has borne out the case against him; for no State has ever constituted itself on his principles or made it possible to live according to his commandments: those States who have taken his name have taken it as an alias to enable them to persecute his followers more plausibly.
It is not surprising that under these circumstances, and in the absence of any defence, the Jerusalem community and the Roman government decided to exterminate Jesus. They had just as much right to do so as to exterminate the two thieves who perished with him.

Sādhanā : The Realisation of Life http://www.spiritualbee.com/spiritual-book-by-tagore/ (1916)
Context: Of course man is useful to man, because his body is a marvellous machine and his mind an organ of wonderful efficiency. But he is a spirit as well, and this spirit is truly known only by love. When we define a man by the market value of the service we can expect of him, we know him imperfectly. With this limited knowledge of him it becomes easy for us to be unjust to him and to entertain feelings of triumphant self-congratulation when, on account of some cruel advantage on our side, we can get out of him much more than we have paid for. But when we know him as a spirit we know him as our own. We at once feel that cruelty to him is cruelty to ourselves, to make him small is stealing from our own humanity...

"The White Goddess," lines 18–22, from Poems and Satires (1951).
Poems
Context: But we are gifted, even in November,
Rawest of seasons, with so huge a sense
Of her nakedly worn magnificence
We forget cruelty and past betrayal,
Heedless of where the next bright bolt may fall.

Source: The Gifts of Imperfection: Let Go of Who You Think You're Supposed to Be and Embrace Who You Are

Section II, Chap. III.
The Theory of Moral Sentiments (1759), Part II

As quoted in Our Day of Empire (1954) by Louis Obed Renne, p. 180.
Attributed

The Midwich Cuckoos (1957), ch 19 - p.200 [Joseph]

Aleph (2011)