I’ll say again that it’s my opinion that Obi Wan and Yoda are not telling Luke to kill Vader and the Emperor. The way it’s worded, Obi Wan is essentially saying that Luke has to be willing to kill his father, not that he has to. Luke is saying “I can’t” do it which is placing limits on himself about what he can do if the need arises. The words Yoda uses are “confront” and “conquer” which are not the same thing as “kill” and “assassinate.” Yoda tells Luke that he needs to confront Vader again in order to become a Jedi. What does that mean?
I think it’s closer to what they call the Jedi trials to become a Knight in the prequels. It’s some kind of spiritual test. Exactly like when Luke faced the vision of Vader in the cave in Dagobah in ESB. We see the progression: facing Vader in the cave and failing because he brought his fears and weapons, then again failing when confronting the real Vader in Cloud City for the same reasons and lack of preparation, and then finally in ROTJ he is actually ready, he’s made some peace within himself that Vader is his father and he has the proper training and attitude. But he has to face his fears again in order to pass the test and truly become a Jedi. Whether that means killing Vader or something else happening is unclear and dependent on how it plays out.
Moreover, what Luke does in the throne room is IRRELEVANT to the Battle of Endor. That’s made very clear. “Soon I’ll be dead, and you with me.” If all goes according to Luke’s plan they’re all going to get blown up anyway. The real battle is spiritual, it’s over Luke’s soul and Vader’s soul. If he did run in there and manage to somehow kill both of them, there’s still the whole battle left. Yoda and Obi Wan must have known that. If you say, oh well, Revenge of the Sith shows them trying to kill Palpatine and Vader and failing, I would say exactly! So they know that’s a nonstarter especially for someone who’s not even a full Jedi like Luke. They’re not sending him to die necessarily but they are sending him for a spiritual confrontation of some kind.
I think Return of the Jedi in general is really misunderstood and that all three original movies are equally good.
With that said, in the Obi Wan show there isn’t really a good reason for Obi Wan not to kill Vader other than just repeating the same pity that he had in ROTS.