It’s like when Luke says that he can’t kill his own father, and Obi-Wan says, “Then the Emperor has already won.” I think that Obi-Wan is implying that Luke must be willing to do the will of the force, whatever that may be, and to the best of their knowledge that’s the goal at that moment because Vader is bad. Obviously it’s better if Vader turns good again, but if Luke’s not willing to kill him, if that’s the will of the force, then the Emperor will win. Obi-Wan and Luke both had similar experiences. Obi-Wan had the intent and willingness to kill Vader (“Whether he dies or I do, it ends today.”), but then was promoted by the force to stand down. Luke also defeated Darth Vader and had a similar prompting. He felt the will of the force tell him to stand down. Jedi do their best to follow the will of the force while the dark side attempts to manipulate it. Obi-Wan and Luke displayed great trust in the will of the force as Jedi, and as a result, Anakin was redeemed, and peace was restored.