It was said that you would destroy the Sith, not join them. Bring balance to the force, not leave it in darkness

"A flash of metal through the sky, and Obi-Wan felt the darkness closing in around them both. He knew that ship: the Chancellor's shuttle. Now, he supposed, the Emperor's shuttle. Yoda had failed. He might have died. He might have left Obi-Wan alone: the last Jedi.
Below his feet, Darth Vader burst into flame. "I hate you," he screamed. 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 Jedl Knighthood, and sworn himself to the Jedi forever. In the end, he was still Ob-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.
He turned and walked away. After a moment, he began to run. He began to run because he realized, if he was fast enough, there was one thing he still could do for Anakin. He still could do honor to the memory of the man he had loved, and to the vanished Order they both had served."