This could be interpreted as Obi-Wan showing either pity or mercy for his old friend. Despite everything they have been through, the Obi-Wan Kenobi show has firmly established that the Jedi still deeply cares for his former Padawan and remembers him as the great ally he's had for many years.
Although Vader was the only one to survive this battle, it was clear that he was not the victor. By letting Vader win, Obi-Wan had ensured that his enemy would never overpower him.
If he killed "Ben Kenobi," it would have been way too easy. So, Vader let him go so that Obi-Wan could regain his full strength because what Vader really wanted was to prove that he was more powerful than his master ever was -- and offing "Ben Kenobi" would have been a massive letdown.
If we go by the need to rationalize what we saw, Obi Wan either felt Anakin deserved to die in pain over however long it would take, or didn't really want to kill him. The obvious reason is that Anakin needed to survive in order to become Darth Vader.
For years, Vader hated Obi-Wan, having been struck down by his mentor and left for dead while also believing the Jedi had worked to turn his true love Padmé against him.
Why Obi-Wan Let Vader Live AGAIN After Vowing to End Him - My Analysis
Who was Obi-Wan's true love?
The biggest reveal is an ongoing tragic love story involving Obi-Wan (voiced on the series by James Arnold Taylor). Back when he was an padawan, he got to know Satine Kryze (Anna Graves), future Duchess of Mandalore. We don't know how far things went between them, but they were in love.
Obi-Wan only believed Anakin could be saved when Luke did on Endor. So, at that moment on Mustafar, Obi-Wan should have finished Anakin, but he didn't have the courage to do it because ironically he had an attachment to Anakin.
David Porter provides some great philosophical context to why Obi-Wan would not regret leaving Anakin alive, as it was obviously “the will of the Force” that Obi-Wan be unable to finish him off, and “the will of the Force” that Anakin live on to kill Sidious himself in the future.
Thanks to that scene, there are now two possible explanations for the sudden jump in Obi-Wan's physical age: it was a result of the Dark Side of the Force being used against him, or it was a result of the burns he suffered.
In a way, Obi-Wan and Anakin were like orphaned brothers, both of them left parentless after the death of Qui-Gon Jinn. As he turned to the Dark Side and became Darth Vader, Anakin came to hate Obi-Wan, but never stopped respecting him.
Anakin Skywalker and Obi-Wan Kenobi have a unique relationship in Star Wars: Attack of the Clones. The master and Padawan have profound mutual respect and admiration but are not afraid to question each other in times of conflict or uncertainty.
Obi-Wan Kenobi, after all, is still a Jedi master, and his decision to spare Anakin is informed by the threads of the Force, including the political implications of killing Darth Sidious' apprentice. In fact, this also explains why Obi-Wan let Darth Vader slay him in A New Hope.
Vader responded “I am not your failure, Obi-Wan. You didn't kill Anakin Skywalker. I did.” This allows Obi-Wan to finally accept that Anakin is truly gone and that there was nothing he could have realistically done to save him.
Kylo and Rey (Daisy Ridley) also blasted the Imperial Crusier open trying to Force pull the Lightsaber away from each other, so clearly, Kylo Ren is an extremely strong Force-wielder. Nonetheless, it can be said that Darth Vader is the more powerful master of the Force for one key reason: his temperament.
The realistic answer seems to be, yes, Anakin in his prime could have defeated Luke Skywalker (in the current canon) if they battled one-on-one. The main reason behind this is Luke's lack of formal Jedi training, at least when it comes to combat.
Late Vaders total strength is a balance of variables. Age, his suit and his de-limbing lower his strength, but obviously his pain, anger and hate connect him to the dark side and make him more powerful.
Though Obi-Wan was often an important ally for Anakin, the young Jedi could never see past his envy, even going far enough to allow himself to believe that his master was having an affair with his wife. In the end, Anakin's own jealousy was his undoing and the reason that he would never surpass his master.
The canonical comic Star Wars: Darth Vader #7 confirms the younglings' deaths caused Anakin grief, self-loathing, hatred and pain, fueling his descent even further into the dark side.
When Anakin was presumably destroyed on Mustafar in Revenge of the Sith, Obi-Wan took the lightsaber as a way to preserve and show respect for the friend he once had before Anakin was corrupted by the Sith. Since the saber had not yet been bled by Anakin, it preserved the part of its wielder that was good.
Obi-Wan knew that even if he defeated Darth Vader, he could never have fled the Death Star, so he accepted his fate and let Vader's lightsaber strike him down.
Obi-Wan couldn't help wishing Anakin the happiness he had been denied, and therefore, he decided to keep the marriage between Padmé and Anakin a secret. There is a tragic irony to this because, in truth, Obi-Wan shared Anakin's greatest weakness: attachment.
