ROTS is about Yoda and Kenobi losing and facing defeat and becoming outcast. We see that it has hit Kenobi hard. This series is about Kenobi finding himself again and once again becoming a legendary Jedi Knight. As Anakin said in ROTS when he had Dooku, killing is not the Jedi way. Kenobi isn’t out to kill Vader, just out to confront him and bring the chase to a close. He has already saved Leia. So he has no purpose there once he has stopped Vader. What is killing him going to do? There is still Palpatine. There are still the inquisitors. And the words of Padme may be ringing in his ears, “there is still good in him.” Kenobi tells Luke that he once thought that as well, and this may be that moment. Even though Vader has just said he killed Anakin, Kenobi may not be ready to accept that yet. There are lots of ways to read this that make sense in light of everything else we have in the main Saga films. We don’t have to assume that Kenobi missed the perfect chance to kill him. Killing a being who is just one of many evils in the galaxy won’t change the evil of the emperor. And we don’t know what images of the future Kenobi might have seen that leaving Vader alive is the best option. We know, having seen the OT, that the best option is for Vader to live for Luke to confront. This moment of mercy by Kenobi leads to the redemption of his friend 13 years later.

I think considering this a plot hole is really disingenuous to what we know of Kenobi and the OT.