Author's note I didn't want to write this, as Matthew Stover already did a splendid job. But I did it anyhow at the behest of my faithful reviewer Shorty51. Dialogue taken from the book, not the film. I think it may be a bit different.
The pain of it nearly flattened Obi-Wan. The Force was shrouded by his grief. How could it have come to this? His flying blade parrying the mirrored blue one of his one-time Padawan and best friend.
There was blinding hate in Anakin's eyes. It was the counterpart of the grief that shone in Obi-Wan's.
It was said by many that Anakin was one of the most powerful Jedi. Obi-Wan even recalled saying it himself a time or two. And here he was.
Anakin was strong, to be sure, but his style was based on impulse and assumption. Obi-Wan knew this. Anakin used the Force strongly as well, but not always wisely.
Obi-Wan parried everything dealt, hoping his defensive position and his pleading eyes would bring back the Anakin Skywalker he knew and loved.
He wanted to shout encouragement to his former apprentice, wanted to give pointers. But he said nothing. Nothing he wanted to say was appropriate.
They battled over the scorching orange river. Obi-Wan didn't feel the singe; ignored the burns on his hands; ignored everything as he let the Force dictate his footwork as his reflexive parry.
Then Obi-Wan knew the end was near. Anakin formed a plan. A very stupid plan. The look in his eyes was one of crazed invincibility. Obi-Wan wanted to scold him.
And then, there he lay, limbless. Obi-Wan hadn't acted consciously, though he knew what it was he had done. What he had been forced to do. But there he lay, possibly the greatest Jedi born, the hope of the galaxy and Obi-Wan's friend.
There was pain in Anakin's eyes, but it was overshadowed by a growing hate. Obi-Wan marveled briefly how the hate could grow.
"You were my brother, Anakin. I loved you, but I could not save you." His voice was heavy with guilt and sorrow. He wanted above all things to see the hate evaporate, to take the boy in his arms and comfort him.
"I hate you." Anakin screamed as he caught fire. His voice indicated that there was no love left in the broken body.
Obi-Wan stooped and grasped Anakin's saber hilt, shaking it loose of lifeless, disembodied fingers.
He turned, walked away.