I don't disagree with any of that, but apparently Lucas did lol. Remember Luke's conversation with Obi-Wan's ghost in ROTJ? An excerpt:
Luke: I can't do it Ben.
OWK: You cannot escape your destiny. You must face Darth Vader again.
Luke: I can't kill my own father.
OWK: Then the Empire has already won. You were our only hope.
So in Obi-Wan's (& presumably Yoda's) view it would have been OK for Jedi Luke to kill Vader in battle (and carry patricide baggage for the rest of his life), but it wasn't OK for Obi-Wan to defeat him in battle fairly and then finish the job? I think any other Jedi would consider it a mercy to spare their former friend of his servitude to the Emperor and the physical pain he has to endure on a daily basis. Also, while Vader was defeated he was far from helpless, a lesson Obi-Wan would have known from the last time he beat him and thought that.
IDK, I don't expect agreement on this but despite how much I enjoy Ewan's work in the role IMO this show ultimately besmirches Obi-Wan's character. He's seen firsthand the death and destruction that Vader has caused and yet in the end he doesn't man up and do what needs to be done. Better to have never made the show than to see a beloved character screw the pooch yet again.