Cruelty is an evil thing befitting least of all a man, and is unworthy of his spirit that is so kindly; for one to take delight in blood and wounds and, throwing off the man, to change into a creature of the woods, is the madness of a wild beast. For what difference does it make, I beg of you, Alexander, whether you throw Lysimachus to a lion, or yourself tear lion to pieces with your teeth? That lion's maw is yours, and yours its savagery.
Thus we see here a linking of cruelty and beastliness; he not long after goes on to note that the most terrible form of insanity is when cruelty is so far advanced that killing people becomes delightful. He does not seem, however, to regard the two as exactly the same, although the precise boundary between the two is not clearly drawn.
He will also consider the nature of the vice itself, arguing that it consists in "harshness of mind in exacting punishment" and is an "inclination of the mind toward the side of harshness [atrocitas]".
When Aquinas considers the vice (2-2.159), he will be heavily influenced by Seneca's account, although he will as usual have others in view, as well. Linking crudelitas (cruelty) with cruditas (rawness), he suggests that the primary experiential marker, so to speak, of cruelty, is a kind of bitterness opposed to the sweetness of mercy and clemency. Since Aquinas takes there to be a distinction between mercy (misericordia) and clemency (clementia), with mercy being more concerned with positive action and clemency more with restraint from negative action, he clarifies that cruelty is more properly opposed to clemency, which is a potential part of temperance. He notes, though, that in practice we tend to conflate mercilessness (immisericordia) and cruelty due to the similarities in the vices.
Aquinas will sharply distinguish cruelty from savagery (saevitia) and brutishness (feritate). They are motivated by delight in suffering, and thus the motivation is not particularly rational; they are therefore forms of beastliness (bestialitas), which is a sort of removal of reason from action. But, following Seneca, he takes cruelty to be rational by its very nature; it is based on the notion of penalizing someone, and gets its viciousness from being a punitive excess. It therefore is not a matter of beastliness but wickedness/malice (malitia).
Discussions of cruelty in the modern age will end to drop all of these distinctions again; most often, probably, by 'cruelty' is meant what Aquinas would call 'saevitia' or savagery, although it is often treated as what Aquinas would call 'immisericordia'. Thus it is generally less clear in modern discussions whether we are dealing with the kinds of actions that are motivated rationally or those that are motivated nonrationally, and whether we are concerned with something opposed to restraint of bad action or something directly opposed to positive good action. This is somewhat ironic, since the lack of clarity has been accompanied by a massive expansion in the use of the term. In ancient and medieval periods it comes up, particularly in the discussion of tyranny, but it is a very secondary matter. In the modern age, however, it has arguably become a central moral concept. This was due, I believe, to the development of movements in the nineteenth century to ameliorate the treatment of animals; thus the 'Society for the Prevention of Cruelty to Animals'.
Perhaps the single best modern discussion of cruelty, however, is Philip Hallie's "From Cruelty to Goodness". The kinds of cruelties he has in mind are those inflicted in the institution of slavery or in the Holocaust. This kind of cruelty involves the attempt to degrade its victims and destroy their dignity as human beings. Hallie argues that cruelty in this sense is opposed not primarily to kindness or to liberation but to hospitality.