As shown in the “Obi-Wan Kenobi” show and mentioned by Rick McCallum, Obi-Wan fully believes he’s killed Anakin after their duel on Mustafar. He severs his legs and leaves his body burning in lava. Just watching the film, it’s easy to assume that Obi-Wan simply doesn’t have the stomach to strike him down when he’s so helpless and pitiful, but that’s not necessarily the case.
In the novelization of “Revenge of the Sith,” which isn’t technically canon anymore, Obi-Wan actually thinks that putting Anakin out of his misery would be too merciful. It also suggests that he detects Darth Sidious’ approach and decides to leave Mustafar quickly, fearing that lingering even a moment longer could put him in danger. He never suspects that Anakin’s rage and Palaptine’s dark side powers could be enough to keep him alive. The cyborg wraith that rises from the (literal) ashes is beyond what Obi-Wan could have anticipated in the moment.
