"You have to mean it, Harry." Who said it: Lord Voldemort Book/film: The Order of the Phoenix If Bellatrix had just killed the last hope of a family that you had left, would be able to resist performing the Cruciatus Curse on her? The point in magic is that you have to use your emotions in order to perform spells because you need conviction; look at the happy memories you need to produce a Patronus. Despite Harry's understandable anger, he doesn't have the ability to torture someone, regardless of the crimes that they have committed. This is what sets Harry and Voldemort so far apart from each other. Voldemort is able to perform the Dark Arts with ease because of his desire to inflict pain and devastation on others; he lives and breathes for that power and sadistic pleasure. He killed Harry's parents, friends and whole load of witches, wizards and muggles and yet Harry can only manage 'expelliarmus.' But that's all he needed.