During the course of The Dark Half we learn that the killer is actually George Stark, an alter ego that Thad Beaumont invented in order to publish books not to be brought in relation with his name. When Thad decides to put an end to this alternate carreer, this alter ego turns real, though more as an incarnation of Alexis Machine, the antihero from George Stark's books. He seems to embody something like Thad's dark evil side and tries to push Thad into writing a new story about George in order to make him completely real. But then there is also the story about the "tumor" that Thad got removed from his brain when he was a child, which is revealed to actually have been the remnants of Thad's unborn twin that Thad absorbed into himself pre-birth.
But I am not completely sure how to bring those characters into relation. If Thad's evil side was bound to and removed with his unborn twin and now comes to life, then why is this George Stark whom Thad only invented years later, and why didn't it break out earlier? But if it is George Stark representing Thad's hidden evil side and created by him, turning to real life after Thad's attempt to metaphorically "kill him", then how is he related to the twin? Why even need the story about the twin then if he isn't related to Stark at all (though the sparrows, which are repeatedly used as "soul escorts" in the story, appearing at the surgery suggest that even back then the twin already had some kind of soul or a part of Thad's soul)? So what is the connection between George Stark and Thad Beaumont's unborn twin and what is the real nature and background of the killer? Did I miss anything in the movie or is this explained further in the novel?
And as a side question, why is it that George Stark's real incarnation is actually more like Alexis Machine, Stark's book hero, but wants to turn to full life by Thad writing about him, while Stark has never been a book figure but someone writing books? (Though, that may just be a minor iconsistency/mixup, but could also have a better explanation or be related to the above questions.)