Lord Sidious gained everything: political power, a powerful apprentice, and more thanks to the success of the Clone Wars project. Lord Vader, on the other hand, lost everything: his beloved wife, his body, and his fellow Jedi when he betrayed them. His only hope was his ambition to betray Sidious, who reigned on the throne, and become the highest power in the empire. We see this in the scene where he seduced Padme before fighting Obi Wan in Mustafar. However, Padmé refuses and loses hope of living in the face of a changed Anakin. In summary, Anakin killed Padme. When Vader hears about it from Sidious, he is overcome with grief and anguish. At the same time, he is angry and distressed at himself for killing her. After the operation, Lord Vader destroys everything in the room. Sidious is relieved that he was not killed and is very glad that the mechanized Vader is still angry. Smiling behind Vader's back.
But in fact, Sidious did not expect Lord Vader to fall so far into the dark side. Anakin Skywalker's thirst for political power, his mastery of the Sith essence, his ascension to mastery, and so much more, was nearly foiled by his fight and defeat against Obi-Wan, which nearly killed him. In response, Lord Sidious uses mysterious Sith medical techniques to revive Vader, who is trapped in the illusion that Padme is safe. Although he expected Padme to lose hope of living, the fact that he promised Anakin that he would save her was intended to cause Anakin himself to lose interest in Padme because he was disappointed in her refusal to walk the path to power with her. However, Sidious is surprised when a disappointed Anakin kills Padme. He thought about hiding it from the resurrected Vader, but in the end, he announces it.
It made Vader realize that his only purpose in life was to gain power and rise to mastery. When Vader learns that he actually killed Padme, he understands that.
That's why he didn't kill Sidious. That is, he knew he needed to keep Sidious alive until the time was right. Sidious himself saw that treacherous ambition. So, pretending that Vader would not see it, he amplifies his thirst for power to make Vader a stronger pawn. And to avoid being easily betrayed, he did not show surprise.
What Sidious fears most here is that Vader will realize he is being deceived. That's why I had him fitted with a respirator to prevent him from spiraling out of control and to make it easier to kill him with an electric shock. Vader himself realizes that he cannot easily deceive Sidious.
Thus, the ideal protégé, the work of a Sith, is complete.