I suggest looking up critical essays at your local or school library and online in scholarly journals.
I recommend the book"Twentieth Century Interpretations of The Adventures of Huckleberry Finn" Edited by Claude M. Simpson, which is a collection of essays. In Gilbert M. Rubenstein's essay "The Moral Structure of Huckleberry Finn," he writes, "Twain makes a sharp distinction between two types of cruelty - the deliberate callousness of the hard-hearted, and the unintentional or thoughtless indifference of the normally kind-hearted" (58). He goes on to discuss the Duke and Dauphin's deliberate cruelty towards Jim and the Wilks girls. This essay would be a good one to look at for your research.
I also recommend the following books of critical essays:
"Coming to Grips with Huckleberry Finn: Essays on a Book, a Boy, and a Man" by Tom Quirk and "Mark Twain's Huckleberry Finn: Problems in American Civilization" Edited by Barry A. Marks.