When pledging his allegiance to Darth Sidious, Anakin states: "I will do whatever you ask... just help me save Padme's life, I can't live without her."
However, after learning of Padme's death, Anakin can clearly 'live without her'. The whole reason he joined the Dark side in the first place was because he believe the Jedi were too weak to save her, but as the Dark side has also failed, surely he could decide to turn back to the light side?
According to the film's Junior Novelisation, the Emperor clearly feels that the opposite is true. The knowledge that he killed Padmé will feed Vader's anger, fear, pain, frustration and despair, all negative emotions that lead you toward the Dark Side of the Force, rather than away from it:
The helmet turned, as if the burned and weakened eyes within were scanning the room, adjusting to the screens in the helmet that magnified and intensified everything so that they could pretend to see. “Where is Padmé? Is she all right?”
And now, the final touch, Darth Sidious thought. The words that will bind him forever to the dark side. And they won’t even be a lie, not really. “I’m afraid she died,” he said, putting a hint of gentle sorrow and reproach into his voice. “It seems that in your anger, you killed her.”Vader groaned in protest. And then he screamed. Leaning forward, he broke the straps that had held him to the table, and screamed again. Things imploded and flew around the room — spare parts, droids, anything that wasn’t tied down — as Darth Vader gave expression to his pain and despair.
And while Darth Vader screamed, Darth Sidious smiled. His apprentice was his, now. Forever.
The film's Official Novelisation say essentially the same thing, except with fancier words:
You killed her.
You killed her because, finally, when you could have saved her, when you could have gone away with her, when you could have been thinking about her, you were thinking about yourself…
It is in this blazing moment that you finally understand the trap of the dark side, the final cruelty of the Sith—
Because now your self is all you will ever have.
And you rage and scream and reach through the Force to crush the shadow who has destroyed you, but you are so far less now than what you were, you are more than half machine, you are like a painter gone blind, a composer gone deaf, you can remember where the power was but the power you can touch is only a memory, and so with all your world-destroying fury it is only droids around you that implode, and equipment, and the table on which you were strapped shatters, and in the end, you cannot touch the shadow.
In the end, you do not even want to.
In the end, the shadow is all you have left. Because the shadow understands you, the shadow forgives you, the shadow gathers you unto itself—
And within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker. Forever …
