Not to be an annoying "in the book" person...but in the book it's shown that Joffrey has always been morally clueless. People saying he was just coming into it or that it's a result of stress from trying to be king are mistaken.
Once after learning a kitchen cat was pregnant, Joffrey killed the animal and cut open its belly to see the kittens inside. He showed one of the unborn kittens to his father. Robert was so shocked and angry he hit the boy so hard it knocked out two of his baby teeth. Cersei also remarks that Robert would have beaten the boy if she had allowed it.