Mostly, I think, that books turned movies are always butchered and desecrated. It is practically imposible to fully captured the vision of a book as everyone who loved said book will always have experienced it differently. Here are a two books, however, that I feel where done a terrible injustice.
Pride and Prejudice by Jane Austen, the film done in 2005 starring Keira Knightley and Matthew Mcfadyen was a so far away from the original novel that it might as well have been an original idea with name of the book slapped over it. However, I must shamefully admit to having liked the cinematography, soundtrack and style of the movie, that part was gorgeous.
Harry Potter and Prisoner of Azkaban by J. K. Rowling, the movie was absolute murder of the book itself. I count book 3 of the Harry Potter series as the best but the movie was a pale shadow of the emotional intensity of the book.