Another person is nearly an equal to you (by definition) and so a type of mirror by which you see yourself, making the things you notice in them, such as their feelings, largely your own. To cause another person to suffer, then, would be to cause suffering in yourself, for in spite of the distance that stands between us, we, and even the universe itself, are bound together by empathy, a condition of being-one that transcends gulfs. Since we can't directly cause ourselves to suffer, for reasons described below, in order for cruelty to occur, a person must make-other, over and above any similarities.
Consider your own hand; a moment ago, you were so much at one with it that you weren't even thinking of it; now that you're looking at it, it becomes possible to do something to it (rather than with it), for in spite of the empathy that still binds you to it, it's been separated from you.
Suppose it occurs to you to take advantage of your hand's objectivity and stab it with a knife. The moment you think of this, you recoil, rather as you would if a loved one were threatened with a knife. What's more, you have an almost physical sense of not being able to do it, as though a magnetic wall stands between the knife and your hand.
A will cannot act on itself, for its willing is [at one with] its willing. (Yet nothing whatsoever keeps a will from acting on itself.)
Now suppose you rest your hand on the table and stare at it until it becomes less your own and more something foreign like a lump of wax. When this happens, it becomes very easy, insofar as you preserve that state of mind, to bring the knife down on the hand, for the hand is no longer unified with your will but at a distance from it. One might say that your hand has been dehumanized, stripped of its freedom and left at the mercy of yours.
This same process occurs when we abuse our fellows; we detach from them and consider them lifeless objects to be treated like machinery. And unlike your hand, which will scream out in protest the moment you infringe on its rights (for you immediately reclaim your hand), the cries of other people are muffled by all the differences that lie between us. Class, nationality, gender, and religion--these are all barriers to our humanity, walls that turn us into lifeless objects devoid of freedom. Some labels are worse than others; with the right one, it's possible to create such a distance that you see another person as scum to be wiped from existence. For the person so disposed, their actions don't seem cruel; they're not aware of the pain they're causing any more than you and I are aware of hurting a stick of wood that we snap in half. To the person on the receiving end, the suffering is very real, and the name for what they're subjected to is cruelty. Every victim has one hope, and that hope is to become human like their oppressor, for no person in this world can harm himself.