Se você se lembra da luta, ele tentou FALAR Anakin de tirá-lo de uma posição desvantajosa que resultou em ele ser cortado:
OBI-WAN: It's over, Anakin. I have the high ground.
ANAKIN: You underestimate my power!
OBI-WAN: Don't try it.
E mais tarde:
OBI-WAN: You were my brother, Anakin. I loved you. [leaves]
Então, ele não queria a morte de Anakin.
No que diz respeito a "salvar" Anakin da gravação, não parecia haver qualquer possibilidade disso:
OBI-WAN looks in horror as ANAKIN becomes engulfed in flames. OBI-WAN can't watch him as he struggles to climb the embankment, covered in flames.
Obi-Wan razoavelmente assumiu que Anakin já estava morto / morrendo irreversivelmente.
Para abordar o "Por que não acabar com ele" - isso foi respondido na novelização de " Revenge of the Sith " de Matthew Stover: Primeiro, porque ele não teve tempo devido à chegada imediata de Darth Sidious. Segundo, porque não era não o caminho Jedi.
Obi-Wan looked down. It would be a mercy to kill him.
He was not feeling merciful.
He was feeling calm, and clear, and he knew that to climb down to that black beach might cost him more time than he had.
Another Sith Lord approached.
In the end, there was only one choice. It was a choice he had made many years before, when he had passed his trials of Jedi Knighthood, and sworn himself to the Jedi forever. In the end, he was still Obi-Wan Kenobi, and he was still a Jedi, and he would not murder a helpless man.
He would leave it to the will of the Force.