Did Obi-Wan regret leaving Anakin alive?

Question

Just watched the RotS duel scene, right after...

Obi-Wan leaves Anakin for dead and Palpatine salvages what's left of him.

Once Anakin was incapacitated, Obi-Wan had the upper hand and the capability to finish him off with his lightsaber, but presumably the belief that the volcanoes of Mustafar had made him good as dead and/or the love he still had for him prevented him from doing so.

1. Is there any material depicting the moment that it was first revealed to Obi-Wan that he hadn't finished him off?

2. Is there any material suggesting he regretted not doing so?

Answer 1

We don't know whether or not Obi-Wan came to regret his decision to let Vader live, but we do know that he definitely wanted Luke to kill Vader. In fact, he explicitly says that if Luke doesn't kill Vader, the Emperor will win:

OBI-WAN
You cannot escape your destiny. You must face Darth Vader again.

LUKE
I can't kill my own father.

OBI-WAN
Then the Emperor has already won. You were our only hope.
- Return of the Jedi

However, Obi-Wan was obviously and completely wrong about that, because Luke's mercy towards Vader when Vader was defenseless is exactly what saved Luke from the Dark Side and led to the Emperor's defeat. And Obi-Wan is clearly quite pleased to have been proven wrong:

And as DVK's answer here states, when Obi-Wan left Vader maimed and helpless on Mustafar, he made a conscious decision to let Vader live:

Obi-Wan looked down. It would be a mercy to kill him.

He was not feeling merciful.

He was feeling calm, and clear, and he knew that to climb down to that black beach might cost him more time than he had.

Another Sith Lord approached.

In the end, there was only one choice. It was a choice he had made many years before, when he had passed his trials of Jedi Knighthood, and sworn himself to the Jedi forever. In the end, he was still Obi-Wan Kenobi, and he was still a Jedi, and he would not murder a helpless man.

He would leave it to the will of the Force.
- Novelization of Revenge of the Sith

As this passage makes clear, Obi-Wan didn't really consider killing Vader now to be a viable option - it would have gone against everything the Jedi stood for, and the whole point of Obi-Wan's fight with Vader had been Vader's betrayal of the Jedi and their principles. If you defend your principles by violating them, you're a hypocrite, and you do more harm to your position than any opponent ever could. If you avenge the murder of defenseless people (like the younglings, or Sand People women and children) by murdering a defenseless person, you have no integrity and might as well join the bad guys. In short, Obi-Wan knew he couldn't kill Anakin because if he did, he would become Anakin.

Furthermore, Obi-Wan's decision wasn't entirely a choice between killing Vader or letting Vader live; the choice is actually between killing Vader or letting the Force decide. Keep in mind that one of the most frequently used words in the Star Wars saga is "destiny". The Jedi (and even the Sith, in most cases) know that destiny, more than personal decisions, is the determining factor in the course of events. Obi-Wan knew that Vader's story would play out exactly as it was supposed to, and nothing that Obi-Wan did would change that. Although he couldn't see it at the time, in the heat of the moment, Anakin's fall didn't change the fact that he was indeed the chosen one, and would ultimately bring balance to the Force - Obi-Wan just didn't realize that the time hadn't come for Anakin to bring balance to the Force, and that was destined to happen decades later, when Anakin's love for Luke triumphed over his loyalty to Palpatine.

My answers here, and here, and here, and here, and here, as well as Thaddeus' answer here, go a bit deeper into the Jedi perspective on killing unarmed opponents - it is especially relevant that Luke's decision to show mercy to Vader at the end of Return of the Jedi is the very thing that keeps him from falling to the Dark Side and inspires Vader to redeem himself and save Luke.

Answer 2

As to your first question, here's an answer I found on this thread, posted by user Jango Fett, who really deserves all the credit.

At the end of Dark Lord: The Rise of Darth Vader, Obi-Wan learns that Anakin still lives when they mention and show Darth Vader on the HoloNet News in a cantina Obi-Wan's in. He knew it was Anakin because he saw the security video of Anakin being called Darth Vader by Sidious. This worried him, and he thought he'd have to hide Luke someplace more obscure. Qui-Gon reassured him that the Anakin they knew is trapped inside of Vader, and that Vader wouldn't set foot on Tatooine ever again, even if only out of fear of reawakening whatever of Anakin was left in him.

Since this story takes begins just before Order 66 is given, and then takes place and ends immediately after the events of Revenge of the Sith, you can sure bet Obi-Wan knew Anakin sruvived before he stepped foot aboard the Death Star.

As for whether on not he regretted not killing Anakin, I am not sure. However, Anakin's fate was foretold in a prophecy, meaning it was bound to happen anyway, and any attempt at stopping it would have only led to it coming true in a different way. Obi-Wan knew this, and he knew killing Anakin would involve too much emotion, which of course is forbidden in the light side.

P.s. Maybe Obi Wan meant for Anakin to survive? He couldn't have left him there out of mercy, because were it not for Palatine, Anakin would have stayed there, limbless, for hours, waiting for death to come. At first, it seems like Obi Wan can't bring himself to kill Anakin because he still loved him, but if such were the case, why would he subject him to such terrible pain? So maybe Obi Wan wanted Anakin to survive, because, as Padme said, he thought "there (was) still good in him". Maybe he left Anakin there to teach him a lesson, in the hope Anakin would become a better person after suffering the same kind of terrible pain he inflected on those jedi children. Just a side note, not a part of my answer.