Terrible things happen to our hero in “Harry Potter and the Deathly Hallows” (Scholastic, 759 pages, $34.99), the last of seven novels in J.K. Rowling’s blockbuster series about a boy charged with saving the wizarding world. He uses Unforgivable Curses for the first time. He has a falling-out with his best friend and lashes out viciously at an old friend of his father’s. His presence endangers everyone around him. He dies, sort of.