A common claim one finds online (and I have heard it put forward by philosophy professors offline, so we know who's spreading it) is that Thomas Aquinas holds that one of the delights of the saints in heaven will be their vision of the suffering of the damned. I came across someone putting it forward just today. Does Aquinas actually say this? As you might guess from the fact that I am devoting a post to it, the answer is No; the claim arises from a misreading of a possible-but-not-probable translation of a passage abridged by someone other than St. Thomas from a very early work by him. What he does say will not please everyone, to be sure, but it's a different claim. And whether one is interested in this particular question or not, it makes for an interesting study in how to interpret certain kinds of texts.
The passage that leads to the claim is from the Summa Theologiae, Supplementum, q. 94, a.1. This is the article on whether the saints view the punishments of the damned. In the most commonly read English translation, the Dominican Fathers edition, the body of the question reads:
Nothing should be denied the blessed that belongs to the perfection of their beatitude. Now everything is known the more for being compared with its contrary, because when contraries are placed beside one another they become more conspicuous. Wherefore in order that the happiness of the saints may be more delightful to them and that they may render more copious thanks to God for it, they are allowed to see perfectly the sufferings of the damned.
First, let's assume that (1) this is a completely correct translation and (2) there is nothing else, outside of question 94, that we need to interpret this correctly. Does the claim hold? And it does not. The claim, remember is, 'one of the delights of the saints of heaven will be their vision of the suffering of the damned', or something very similar. But this passage doesn't say this, even on these two assumptions. It says that the happiness of the saints will be more delightful from comparison to the sufferings of the damned, due to the contrast. This not the same thing as delighting in the sufferings of the damned. This is confirmed by the fact that, just a little further in Supp.94.3ad1, there is a passage that says that rejoicing in what is bad for another is the sin of hatred, and is not found in the blessed. The claim, in short, misinterprets the English translation of the passage.
However, we should check our assumptions, as well. St. Thomas is not writing in English, but in Latin. How good is this translation? Well, it's not bad, but there are a few things that can easily be questioned here. First, the Latin behind the 'more delightful' phrase is complaceat (from complacere/complaceo). One could perhaps interpret this as delighting in something, but complacere, like placere from which i comes, is in Latin often contrasted with delight words; that is to say, it's a word in the vicinity, but it's often employed where you are trying to avoid particular, usually feeling-based, meanings of delight, joy, etc. The essentially idea behind it is that something is pleasing to you, or is acceptable to you, or is satisfactory to you. In moral psychology, for instance, it often indicates a particular kind of acceptance of something. Thus the Latin really says something like 'that the beatitude of the saints may be more acceptable or pleasing to them', i.e., that the beatitude may be something the saints accept more fully.
Maybe, though, you could interpret it as being more delightful -- 'delight' is broad enough that it probably could accept such a meaning sometimes. But a more serious ambiguity arises in the phrase translated above as 'the sufferings of the damned', and it is one that pervades translations of medieval discussions of hell. The Latin here is poenam impiorum. Poena does not generally mean 'sufferings'. It pretty much always means 'penalty' or 'punishment' (both words, in fact, that derive from it etymologically); originally it meant a fine, but in medieval Latin means any kind of punishment. In some contexts, it can mean 'pain' (which also derives from it etymologically), pain being understood as a kind of natural punishment or penalty. One of the extremely common problems with translations of medieval discussions of hell is that in this context translators, perhaps influenced by much later depictions of hell, like to translate 'poena' with 'pain' words, even though neither the word itself nor the context strictly requires it. Frankly, the kinds of discussions of hell you usually get in medieval theological treatises are usually not very vivid; they usually talk about it in a rather abstract way. Sometimes a 'pain' meaning might be intended, but outside a few cases, it's often ambiguous at best whether it is, because they're actually discussing it at a more general level that could include pain, suffering, torment, but does not necessarily do so. What this sentence means would be closer to something like, "Thus, so that the blessedness of the saints might be more acceptable to them and that they may make more fruitful thanksgiving to God, it is given to them to be fully aware of the punishment of the impious." While sometimes 'poena' could indicate some kind of pain, the point in question here is whether the saints know about hell; the passage is simply saying that they are aware of it so that they may more fully appreciate being saved from it (unsurprisingly, since there's no good reason why they would be ignorant of it rather than glad not to be in hell). Later (in article 3), there is a passage that does look at whether they rejoice at it, and the answer is very explicitly that they don't rejoice at the punishment itself, they rejoice in divine justice and their own liberation.
However, we should qualify even this, at least a little bit. Questions about hell as such are Last Things questions, and therefore, in the overall plan of the Summa Theologiae, they are nearly the last things discussed. However, St. Thomas never finished the Summa; it breaks off well before this point in the middle of discussing the sacrament of penance. (A great tragedy, for theology at least, and one that I think is not adequately considered even by many Thomists; in the sacramental sections of the Summa, St. Thomas was heavily reworking his theology of the sacraments in light of his instrumental causation account of sacraments. He never got to unction, orders, or matrimony, meaning that Thomistic sacramental theology on these three sacraments has been defective ever since. Now, the changes are often very subtle, or sometimes just a shift in emphasis, but they are sometimes very significant, and none of Aquinas's older works give an account of these three sacraments that would be adequate given his fully developed instrumental causation account.) Thus St. Thomas never got to the point of discussing the resurrection, hell, heaven, the world to come, in this work. What we have is the Supplementum, which is an abridgement by Fr. Reginald, a close associate, of the corresponding questions in the Commentary on the Sentences from the beginning of St. Thomas's career. Thus we don't actually know what St. Thomas would have said in the Summa on this subject. However, if we go back to the Commentary, it provides a little bit more context. The Supplement here is taking directly and without much change from the three subquestions for Sent. IV d50 q2 a4. But in the Sentences commentary, St. Thomas is not considering these questions as standalone questions, because that's not what you do in a Sentences commentary. There are times when you might diverge quite considerably from Peter Lombard's Sentences in a Sentences commentary, but Aquinas does not do so here; the entire article is just expounding at somewhat greater length on the Lombard's brief comments and collected texts in the corresponding passage in the Sentences. What we are really getting here is an exposition of Peter Lombard; St. Thomas doesn't seem to disagree with it, or else thinks it admits of an interpretation with which he agrees enough that it does not require a more penetrating investigation, and we simply don't know whether St. Thomas himself would approach the matter this way if given the opportunity.
We actually have remarkably little extended discussion from Aquinas on hell. Besides the Sentences commentary, where he mostly exposits Lombard, we have a large number of scattered comments and brief discussions on very specific issues (a few questions in the Summa Contra Gentiles and a number of brief questions in the Compendium Theologiae are probably the most developed discussions, and they are both whirlwind tours of only a few points), and that's about it.