Cruelty is manifested in different ways. People can rightly be judged cruel either for what they do or for what they fail to do, and either for what they feel or for what they fail to feel. The central case of cruelty appears to be the case where, in Locke's apt phrase, one takes "a seeming kind of Pleasure" in causing another to suffer. Sadistic torturers provide perhaps the clearest example of cruelty in this sense: they are cruel not just because they cause suffering (so do dentists and doctors, for example) but because they enjoy doing so. Let us term this sadistic cruelty.
Not all cruel people are cruel in this sense. Some cruel people do not feel pleasure in making others suffer. Indeed, they seem not to feel anything. Their cruelty is manifested by a lack of what is judged appropriate feeling, as pity or mercy, for the plight of the individual whose suffering they cause, rather than pleasure in causing it; they are, as we say, insensitive to the suffering they inflict, unmoved by it, as if they were unaware of it or failed to appreciate it as suffering, in the way that, for example, lions appear to be unaware of, and thus are not sensitive to, the pain they cause their prey. Indeed, precisely because one expects indifference from animals but pity or mercy from human beings, people who are cruel by being insensitive to the suffering they cause often are called "animals" or "brutes," and their character or behavior, "brutal" or "inhuman." Thus, for example, particularly ghastly murders are said to be "the work of animals," the implication being that these are acts that no one moved by the human feelings of pity or mercy could bring themselves [sic] to perform. The sense of cruelty that involves indifference to, rather than enjoyment of, suffering caused to others we shall call brutal cruelty.
Cruelty of either kind, sadistic or brutal, can be manifested in active or passive behavior. Passive behavior includes acts of omission and negligence; active, acts of commission. A man who, without provocation, beats a dog into unconsciousness is actively cruel, whereas one who, through negligence, fails to feed his dog to the point where the dog's health is impoverished is passively cruel, not because of what he does but because of what he fails to do. Both active and passive cruelty have fuzzy borders. For example, a woman is not cruel if she occasionally fails to feed her cat. She is cruel if she fails to do so most of the time. But while there is no exact number of times, no fixed percentage, such that, once it is realized, cruelty is present, otherwise not, there are paradigms nonetheless.
We have, then, at least two kinds of cruelty (or two senses of the word cruelty) and two different ways in which cruelty can be manifested. Theoretically, therefore, cruelty admits of at least four possible classifications: (1) active sadistic cruelty; (2) passive sadistic cruelty; (3) active brutal cruelty; and (4) passive brutal cruelty.
(Tom Regan, The Case for Animal Rights, updated with a new preface [Berkeley and Los Angeles: University of California Press, 2004], 197-8 [italics in original; endnote omitted] [first edition published in 1983])