Knights are supposed to protect women; instead, he rapes one. Knights are supposed to keep their promises steadfastly; he whines and complains and tries to wriggle out of his when it turns out that keeping his promise requires him to marry the loathly lady. Knights are supposed to be respectful of their elders; on his wedding night, he tells the loathly lady to her face that she's ugly, old, and low-born.
