Humans are capable of experiencing joy at the sadistic torturing of another. Is this solely a human emotion? Or can an animal experience that particularly "perverted" pleasure at the torture of an animal simply for pleasure? What about sadistic sexual pleasure, becoming aroused at the sexual "assault" on or the pain inflicted on that of another?
If anyone has any familiarity with writing or research on the matter, I would appreciate it.

The topic of animal emotions is I think inherently difficult. The words we assign to feelings -  like joy or glee are human contructs and it doesn't make a lot of sense (at least to me) that we should apply them to animals. Certainly my view is that we cannot generally know what animals feel emotionally in the sense that I think you mean, although it is quite likely that some researchers (e.g. psychologists working with primates) may disagree.

Having said that there is a lot of research into animal personality, which is usually defined functionally through observed behaviour. So for instance it is often possible to describe variation in aggressiveness among individuals, equally there may be repeatable variation among individuals in sexual behaviour. I see no reason why there may not be among-individual variation in e.g. propoensity to rape among male ducks. Punishment by dominants of individuals that infringe their interests also occurs in many animal societies. Through an anthropomorphic eye this could perhaps be contrued as torture. I think your question though relates not to these functional behaviours, but the motivation for them, and particularly the extent to which (if at all) they are motivated by an emotional reward (e.g., glee, pleasure). Here is suspect that we run into the unknown, in large part due to the reasons given above, but also because even if we can come up with a way to infer animal emotions, there would be serious ethical problems with conducting appropriate experiments.

I very much agree with Alistair's point about the problems inherent in attributing emotional states to animals. However, there are certainly cases of animals behaving in ways that most people would probably describe as cruel or sadistic if a human were doing something similar. The way cats may "play" with mice before finally finishing them off is a well-known example, although I've seen claims that this actually has a practical purpose - supposedly it's safer for the cat to exhaust the prey before moving in for the kill. Even if cats are genuinely playing when they do this, however, I wouldn't like to guess at whether they're aware of (and presumably enjoying in some sense) the suffering of the mouse or simply fascinated by the antics of that small, squeaky, tasty moving object.

There's a considerable literature on violence and cannibalism among non-human primates, and some of what goes on looks awfully like torture. As Alistair said, some of this has to do with enforcement of dominance hierarchies. I've read in a couple of places about a male bonobo (pygmy chimpanzee) in a zoo who was repeatedly bullied by a group of females to the point where they "occasionally held down the male and attacked him, and had bitten off parts of his fingers and toes" (the quote is from a strange but interesting book on the evolutionary origins of human violence, James Miles' "Born Cannibal").

Whether the females enjoyed doing this, or whether it was "strictly business", is an open question. Theoretically, though, one could try to find ways to apply neurological imaging techniques to animals that were engaged in different violent behaviours, and look for indications that they were experiencing pleasure.