Vader had been a Skywalker once, and too many of the traits of that line remained in him. A fierce individuality, a quickness to anger, a certain recklessness, and very great strength in the Force all combined to make a servant of unquestionable value but perhaps too much power.
Palpatine had used Vader as his foremost agent. Vader eventually became the most visible symbol of the Empire. His mask, his stature, his voice, and his powers presented an image that struck fear into most people. They obeyed him, and thus obeyed Palpatine. They did not fear their Emperor, though in reality, they most certainly should have. In Vader, the Emperor had an instrument through which he could project something of his true self, while retaining an image of relative benevolence for himself.
Also, Palpatine admitted to himself, it was simply satisfying to have beside him a living symbol of his victory over the Jedi. Anakin had been one of the brightest and the best of the Jedi, and now here he was, twisted and corrupted, every last trace of goodness eradicated from him. As Vader, he was strong in the Force, but he kneeled to Palpatine, debasing himself and groveling at his Master’s displeasure, and taking his only sustenance from his Master’s praise. Yes, it was better than having a dead Jedi, better by far.
A few years ago, the Emperor had seen the first signs that all was not well with Vader. When Vader had met and killed his old teacher, then discovered he had a son, old, long unfelt connections to the past had stirred in him. It became worse when Vader was given command of the fleet and he used it to indulge his obsession with finding his son. At the time, Palpatine had swallowed his doubts, and perhaps that was a mistake. For, when Vader had finally caught up with his son, his true colors had shown at last.
The Emperor had agreed to try to turn Luke Skywalker, and Vader had agreed to be the one to do it, or else kill the boy. Vader laid a complicated trap, placing Luke’s friends in danger, knowing that the boy would feel their pain and come to their rescue. It worked perfectly, and before long, the boy, full of bravado, faced his father with ignited lightsaber. As he had done so many times before, Palpatine had used the Force to watch his servant. He was keen to take the measure of this boy who inexplicably figured so strongly in his own destiny. As the battle unfolded, Vader tested the boy, urging him to draw power from the dark side by encouraging the emotions that would open him to it. The boy resisted, but by the time he realized he was out of his depth, it was too late for him. That fierce Skywalker determination kept him fighting on, though Vader was by far his superior. Palpatine had been certain that Vader would be forced to kill the boy. The Dark Lord had bludgeoned his way through Luke’s defenses and sliced off his right hand. He cornered Luke, leaving no way out but to turn or die. Then had come the moment that Palpatine even now recalled with dismay and rage. The moment of betrayal.
There is no escape, Vader had told Luke, Don’t make me destroy you. You do not yet realize your importance. You have only begun to discover your power. Join me and I will complete your training. With our combined strength, we can end this destructive conflict and bring order to the galaxy. Treachery! Their combined strength! By the dark side, it would never be. Vader belonged to him! Skywalker was his! It would never be. But betrayal had followed treachery. Vader revealed his identity, and although the boy reacted with anguished disbelief, Palpatine had thought he felt the assertion touch something deep inside Skywalker. Vader had felt it, too. Search your feelings. You know it to be true. Luke. You can destroy the Emperor. He has foreseen this. It is your destiny. Join me, and together we can rule the galaxy as father and son.
Remembering the words, Palpatine began to shiver with anger. Indeed, his visions of Skywalker’s threat to him had opened Vader’s eyes to the possibility of Palpatine’s vulnerability. But there was much to the notion of “destiny” that Vader did not grasp. He would learn that to covet Palpatine’s power was to earn death. From the moment of that betrayal, the Emperor had begun to plot the destruction of Darth Vader. His plan had a great symmetry to it. He would mold events such that one Skywalker would kill the other, and in that act, turn to the dark side and replace him at Palpatine’s side. He would use Vader’s great hope as the very instrument of his murder, and corrupt the son as he had corrupted the father. It was the perfect destiny for both of them.
Except that nothing was certain anymore. The boy was strong, and he had made his servant strong. It would be a risky endeavor. After long decades of being above risk, the Emperor did not like it at all.
The great throne room doors opened, and Vader strode in, preceded by Sate Pestage, flanked by six Imperial guards, and followed by, of all things, a droid. Vader completed the long walk to the throne, then knelt at the Emperor’s feet. This was a ceremonial audience, and Pestage was wearing full dress regalia. Pestage formally announced the presence of the Lord Darth Vader and his request to report to his majesty, the Emperor. Palpatine had designed this formal meeting to remind Vader of his place, and besides, the formality helped Palpatine to mask the anger he felt towards Vader. It would not do to let Vader know how he felt before the moment of his revenge. Even so, Palpatine waited nearly a full minute before acknowledging Vader with a cold sounding, “Rise, and report on the events at Bespin.”