Obi-Wan Kenobi was not truly a sentient man at this moment, just as he had not been when he'd fought the general Grievous on Utapau.
He was the Force, the whirlwind, the light, as he leapt, attacking without antagonisms and missing his planned cut, and rolled onto the near-burning shore. He got a second's look at his former Padawan, familiar, fierce Anakin Skywalker, poised on a tiny relay droid's casing as it hovered over the lava flows that dipped ponderously toward the cataract's lethal edge.
Anakin jumped, a surge of final effort, pain and darkness. Obi-Wan's lightsaber scored a sear down his left side and he fell, ungainly, higher up the shoreline that was beginning to char Obi-Wan's hair. Anakin–Darth Vader, I must know him as the Sith!–crawled a few paces and screamed at the fury of the black sand getting on his hands and against his face. Obi-Wan leveled his lightsaber at the younger man, whose weapon had rolled down the hillside and vanished with a plash of flare into the conflagration.
Obi-Wan the Jedi Master prepared to kill his best friend, the helpless Sith before him. Obi-Wan the man screamed over roaring Mustafar; "You were the chosen one! It was said you would destroy the Sith, not join them! It was you who would bring balance to the Force, not leave it in darkness! You were my brother, Anakin."
"Obi-Wan..." And the Force felt a spark of hope arise in the dying thoughts of the darkened Jedi. Anakin looked up; a silver ship floated by overhead. "Master... help me..." It was Palpatine in that ship, come to steal the child of the Chosen One. He had not come to rescue his apprentice. In that moment, Anakin realized that he'd been coerced, trapped, lied to - he was expendable.
Obi-Wan knew it too. He turned and looked at the fallen apprentice - still his apprentice, still the boy he'd known years ago. In so many ways, Anakin had changed. He was no longer so carefree, so easily trusting or loving. Something had corrupted him. Vader.
Vader was burning. But Anakin could be saved. Maybe.
The Master started back down the hillside, careful not to slip in the ashy dirt. He picked Anakin up with the Force, knowing the pain that touching his skin would cause.
They struggled up each black hill, leaning on the energy that poured from Obi-Wan to his fallen apprentice, spiraling in like star-stuff to a black hole.
All to late. The Chancellor's shuttle–the Emperor's, Obi-Wan reminded himself bitterly–set down on the same platform that the Jedi neared after their seemingly aeons-long hike, the same one holding Padmé's ship, and the two droids. Obi-Wan and Anakin, the Master willful and the Knight shattered enough to do exactly what he was told, crouched behind a great boulder and breathed the cooler, brimstone-scented air.
Palpatine/Sidious took Padmé away in a strange, sophisticated medical vehicle. Clone troopers entered the Nubian skiff, but only to take the droids to Palpatine's shuttle.
After it took off, Anakin went to the platform and fell to his knees. Obi-Wan thought he was going to die. He tried to scream, but all he could manage was a dull whisper. "Padmé!"
Then he collapsed.
Left without other options, Obi-Wan dragged Anakin onto Padmé's abandoned ship. He was not a medical expert, but he could hook Anakin up to the ship's emergency equipment–and he could tell when someone wasn't breathing.
He flew away from Mustafar and sat with disorganized thoughts beside his apprentice, the shushing sound of the respirator he had put Anakin on repeating over and over in his head, until he got the message from Yoda to meet at a certain medical station on a world called Polis Massa, in a galaxy where only two Jedi Masters remained.
He was immediately relieved to learn that he would not be rehabilitating Anakin on his own.