It is assumed that Obi-Wan thought he did kill Anakin on Mustafar. Missing every limb and looked like burned chicken. In this most recent duel, heās going through a ton of emotion, e.g. starting to tear up seeing Anakins face for the first time probably sent him to an area in his mind that is tough for him to reconcile with. Itās like why didnāt Vader just force move the flames again and finish Obi-Wan right then. I think theyāre just really conflicted with each other and neither of them could pull the trigger when they got the chance to.
The RTJ thing is interesting. Iām going to run with the thinking that Obi-Wan thought about his error for 9 years and thought Luke needed to finish the job. Vader canāt keep escaping death. Obi-Wan probably thought about the āI killed Anakinā from Vader harder after their battle and should have finished him off and not even his own son could bring him back to the light. The whole Vader is more machine now and that since Luke wonāt kill Vader, the emperor has already won, makes a bit more sense now from Obi-Wans pov imo.