By the end of the story, Draco finally understood what evil is and how terrible it is. He admitted to himself that this was not his path, that he was not a murderer at heart, but only a spoiled, vain and cowardly magician. Voldemort, living in the Malfoy mansion, constantly mocked Draco's parents. A quiet hatred for the Dark Lord was brewing inside the young man. That's why he didn't betray Harry - to contribute to the victory on Evil, to end the reign of the One Whose Name Must Not Be Spoken.