I happened to notice how furious Darth Vader becomes while arguing with Princess Leia in the beginning of A New Hope. I wondered why this was, as he rarely exhibits emotion. I understand that my theory hinges on what is not canon; Vader doesn't realize that Leia is his daughter until Return of the Jedi, but I think that if he had known that Leia was his child during a New Hope, it would have added a more personal dynamic between them. In short, take this as Alternate Universe as you may.
"You are part of the rebel alliance, and a traitor! Take her away!" Vader turned the memory carefully over in his mind. The almost serene silence of his chamber provided little comfort to him. He subconsciously flexed his right hand, gripping the folds of his dark cape before letting them fall. Never had he expressed such fury so openly, and in the eyes of the public, at that. The stormtroopers hadn't been deaf. That factor wasn't so much an issue; they knew better than to spread rumors about him if they valued their lives. No, what caused such an internal stir was the sheer fact that it had happened. True, he had felt frustration and other measly feelings inside over the years, but he'd managed to destroy them. After all he had lost, nothing more could strike him.
At least, he had believed that to be true. There was one sentiment he could never shake, although he neither admitted to nor cared about it: loneliness. Vader embraced solitude; it helped him to better concentrate on his powers. Relationships were such a distraction, and they proved time and time again to provide nothing but pain in the end. The truth was, however, there was no giving up the ghost. Although he didn't look it, he was still human. He grimaced in disgust behind his mask, and gripped tighter against the memories of isolation, inviting the darkness, both the physical of the room, and the supernatural of the Dark Side, to shield him from them.
Losing Padmé had only been the start of it. He still missed her dearly. There were nights when he would envision her walking beside him in his dreams. Vader never let the sadness at seeing her get to him. It would make him feel weak. Instead, he would enjoy her presence, and remember that strong-willed, beautiful woman he had fallen for, rather than the sobbing, betrayed wretch she had become. His child, on the other hand, was a different matter. In a way, Padmé's death made it easier to think of her. He had his ethereal vision of her to call forth, even though the sight attempted to cut him asunder with thoughts of what could have been. But the flesh of his flesh?
Vader's breath hitched momentarily, and a relaxed breath followed as he slipped into the darkness. His sitting pose still held regality, but it was slightly more relaxed. It tortured him so that he had not been the one to raise his children. It was absurd to the point of hilarity; under it all, Darth Vader was a father. The icy claw of loneliness had seized him the hardest whenever he had mused over what had become of his kin, and that had hardened him even more to the core due to his retaliations. The hardness and the frost had collided when his daughter, his own daughter, had stood defiantly against him. Odd, he had felt more inclined to break her neck than to hug her. Leia was Padmé's little girl, there was no mistaking that. She had her beautiful hair and eyes. She also had her father's fiery temperament, endurance, and calculating mind.
Metal clanged loudly. It would have scared the common man, but Vader merely shrugged, pushing his irritation away. He hadn't concentrated completely on blocking thoughts of his prisoner out. At least it was only a piece of the ship that took the brunt of his unhappiness for the moment. That was why he had become so angry with her. Foolish, arrogant child! She squandered her potential on a lost cause. More importantly, she proudly fought on the opposing side.
Vader knew better than to bang his head against the wall over that; Leia was an adult. True, she shared his genetics, but she wasn't his daughter. He hadn't raised her, hadn't even held her, aside from gripping her arm during the destruction of her home planet. The rebel forces had effectively kidnapped her from him, and brainwashed her. It did justice to have the people who were responsible for her tainted upbringing to be destroyed. With that pass of judgment, he had effectively disowned her. He didn't care what was to happen to her, even if she was to be destroyed. Let Leia die; he washed his hands of it. Still, the way he had made his declaration said otherwise.
The gold orbs that were Vader's eyes popped back open, and narrowed to slits. In those words, he had unleashed all of the pain that he had experienced in her being turned against him. All of his efforts were renounced; he was finished with her. Princess Leia had no father, and Darth Vader had no daughter.