THIS PARALLEL DOES NOT GET ENOUGH CREDIT.
Obi-Wan, in being unable to kill Anakin, because he could never do it, he could never force himself to make that last stroke to end Anakinâs life, he just could not do it, feels as though he let the Emperor win because of it.
Luke faces exactly the same situation and it isnât that Obi-Wan is saying that Luke has to kill him (George Lucas was explicitly clear on that he had to be ready to do it, if Vader tried to kill him, not that he absolutely had to, thatâs not what Obi-Wan and Yoda were saying), but that he has to be ready for it to be a possibility that will come up.
Obi-Wan himself failed at accepting that possibility, even when he went to Mustafar to face down Anakin, to beat him in that fight, even able to cut off his limbs. But he genuinely could not kill Anakin and because of this Vader lived and went on to hurt, kill, or enslave millions of people in the galaxy, because of this, the Emperor won. Because Obi-Wan couldnât set aside his feelings in the moment that was most crucial, even when there was no other way around that Mustafar confrontation. Yoda was right, he couldnât have faced Sidious, either.
And the reason Luke can succeed where Obi-Wan couldnât, where Padme couldnât, where Ahsoka couldnât, is because Anakin cannot face what heâs done and live with it, he cannot face the look on Obi-Wanâs face, on Padmeâs face, on Ahsokaâs face, when they know him, when they knew who he was, when they know what heâd done. Even if they forgive him, theyâll always see who he had been and how he fell and Anakin canât make peace with that and live. But Luke only sees the future, he doesnât know much of the past, he only knows that thereâs still light in Anakin. And Anakin can let go because he doesnât have to face how far heâd fallen from the person he once had been, not and still live.
This was a miracle and one that nobody was happier about than Obi-Wan, but his advice isnât wrong either, from his experience. His inability to face the truth of Anakinâs fall and the consequences of it, his inability to let go of his feelings for Anakin, is what allowed Darth Vader to live. Darth Vader made his own choices, nobody is responsible for them but him, but Obi-Wan certainly feels the weight of, if he had killed Anakin then, if he had been able to do that when Anakin forced his hand, then they wouldnât be in as bad of a situation as they are now.
And that makes sense from his point of viewâand, had Anakin been unwilling to let go of his own hate and rage, had Anakin been unwilling to give up his own life for Lukeâs, itâs entirely possible that Sidious would have deep fried Luke and the galaxy would have been in his clutches still yet, the Emperor would have won again. If Obi-Wan couldnât stop Anakin, if Padme couldnât stop Anakin, if Ahsoka couldnât stop Anakin, itâs entirely reasonable that Obi-Wan would think Luke couldnât stop Anakin, either.
Because Obi-Wan knows who Anakin was, Obi-Wan loved him, and he knows exactly how those feelings can be used against you and evil will triumph.