On the first reading through of the Harry Potter books, the reader will usually become enthralled with the suspense of the story. On the second reading through, a reader will start to realize some of the overall meaning. I have read the series through three times, and the third time showed me how deep and consistent the themes of the book go. It was on the third read through that I saw the importance of the three way struggle between the Order of the Phoenix, the Death Eaters and the Ministry of Magic. It was also when I realized the fact that epistemology is perhaps the basic philosophical issue in Harry Potter.
Another literary theme that becomes more obvious in its frequency and profundity is sadism, the enjoyment of inflicting physical and mental pain and discomfort on others. The type of pain that can be inflicted varies greatly: from the Cruciatus curse which causes excruciating pain able to drive someone insane, to Ron calling Hermione an "insufferable know-it-all". Cruelties are how enemies hurt and frighten each other, but they also are the currency of communication between close friends and allies. Teachers punish their students, and students use small acts of rebellion to embarass their teachers. Pain can be inflicted in ways as obvious as a stomp to the face, or as subtle as pretending affection to inflame jealousy. Every character in the book engages in some form of sadism: even Dumbledore, portrayed as the epitome of goodness, seems to relish the humiliation he inflicts on Cornelius Fudge in Book V.
Many guesses could be made both about the possible reasons for JK Rowling writing so much cruelty into her books, or why such a thing would appeal to readers. In some ways it could just be seen as a way that her writing tends towards realism, despite its fantastic subject matter: sniping, teasing and humiliation are a part of our life, and portraying them is just a natural part of her dialogue. It should also be noticed that while forms of sadism are engaged in by everyone, they are not portrayed as morally acceptable. The characters in the book who are good may engage in some behaviors that hurt others, but they are usually minor, or are the result of provocation, and usually are followed by some remorse. In Book V, Harry Potter, even in a moment of supreme rage, can not actually use the Cruciatus curse on someone, lacking the hatred in himself to do so.
Apart from being either a part of the portrayal of human nature, for both good and evil, I think that sadism is an important philosophical point in Harry Potter, and I think that it is an expression of the central philosophical topic in Harry Potter: epistemology. Pain and humiliation are often inflicted when people are not acknowledging some situation, or on someone else's view of a situation. In Book III, Harry Potter causes his Aunt Marge to swell up after she insults Harry's mother, a woman she knows nothing about. In Book V, Dolores Umbridge forces Harry to mortify his own flesh with the phrase "I shall not tell lies" because he will not acknowledge her reality. Ron calls Hermione an "insuffurable know-it-all" because she seems to be unaware of her own arrogance. Professor McGonagall insults the fortuneteller Professor Trelawney because she thinks Trelawny is a fraud. Dumbledore humiliates Fudge when Fudge is telling a story he knows to be a lie. In all of these cases, and in many others throughout the books, pain and humiliation are used as tools when discussion fails. This is portrayed almost as a natural consequence, a countershove when the natural dialectical process of interacting with the universe is disturbed. Pain is just one of the many twists and turns along the way to learning the truth.
This of course leaves open the question of what exact epistomological reality so inflames the Death Eaters that they dedicate their life to inflicting pain. For most of them, I think it is a matter of hanging on desperately to their reality in which they are special because of their birth. In this case, sadism must be maintained desperately because it has the hard job of keeping reality out instead of letting it in. For Voldemort himself, I think that sadism is a result of his fear of the unknown, and his fear of death: a desperate attempt to torture nature into making sense to him.