Obi-Wan thought that he's far beyond redemption; in hindsight, not really understanding what drove Anakin to the Sith didn't help the situation.
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.
Obi-Wan knew Anakin through and through and understood his overconfidence and arrogance. He knew Anakin would fight with rage and aggression, while he remained cool and calculated. Pairing his understanding of Anakin with his knowledge of Soresu ultimately gave him the upper hand in the battle.
In Episode III, just before they are to set off on their respective missions, Obi-Wan and Anakin share a farewell. Anakin admits to Obi-Wan that he has been arrogant and unappreciative of his training, and apologizes to his master.
By that point Obi-Wan knew that Anakin was un-savable. Anakin needed to be killed to keep him from turning into a galactic dictator. But Obi-Wan couldn't bring himself to kill someone he considered a brother.
He couldn't bring himself to kill Darth Vader, however, because he continues to associate him with the Anakin he trained as a boy. This is also the exact reason why Obi-Wan didn't kill Anakin on Mustafar despite the opportunity a decade earlier in Star Wars: Episode III - Revenge of the Sith.
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.
It is unclear whether Yoda would be more or less forgiving than Obi-Wan but one thing is certain - the former Grand Master would be ready to treat Anakin as an ally, following the events on the second Death Star. In Star Wars: Revenge of the Sith (2005), Yoda never really lost faith in Anakin in the way Obi-Wan did.
He explained to Luke, “I took it upon myself to train him as a Jedi. I thought I could instruct him just as well as Yoda. I was wrong.” His guilt was apparent in Revenge of the Sith, when he explicitly told Anakin in the heat of their duel on Mustafar, “I have failed you, Anakin.
If Anakin was still on Obi-Wan's side, he would have won himself a trip to Utapau and continued to defy orders until he became the ultimate jedi master. Anakin would have still turned to the dark side because the plot said he had to.
It's easy to criticize Obi-Wan for his attachments to Anakin and to others. In truth, his struggle with the same problem as his Padawan simply underscores the impossible ideal Jedi pursue. Love without fear of loss is easy to talk about but hard to achieve, even for the greatest Jedi.
Obi-Wan Kenobi
In his prime, Obi-Wan was one of the most skilled fighters in the galaxy. He even managed to defeat Anakin Skywalker in single combat, although he didn't have the heart to finish him off.
Yes, when Darth Vader turned back to Anakin Skywalker and sacrifice himself to save his son, he dies and Obi-Wan uses the force to connect with Anakin's spirit. In that time Anakin apologizes to Obi-Wan, desperately asking him to forgive him for everything he has done to him and everybody over the years.
The added risk Darth Vader takes in allowing children to live in a situation where he could have killed them proves that he regrets killing Younglings and will avoid harming any more children for the rest of his days, though intense intimidation is a dark strategy to avoid repeating his Youngling massacre.
Yes, his entire character since then was a self loathing tragic monster who has little reason to care about anything. Betraying Mace Windu instantly made him regret his act of impulse, Padme's death and rejection of his actions later on made it stick. To Anakin, he tried to rationalize he had no choice.
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. Those negative emotions also power the dark side, meaning Anakin likely felt stronger as he killed the younglings.
Yoda didn't specifically know how this would go down, or to what degree Anakin would cause it to happen, but he was playing a much longer game by Revenge of the Sith, and that's at least, in part, explained by Yoda's Force connection with Qui-Gon's spirit.
Darth Vader hated Obi-Wan Kenobi because he hated himself. This started when he thought that Kenobi was the one that poisoned Padmé into turning against Anakin when he turned to the dark side. But the truth was that Anakin was merely projecting his own self-hatred toward Obi-Wan.
Obi-Wan knew it was only a matter of time. So, he made the difficult decision to use his learnings and become one with the force, rather than let Vader decapitate him. Who knows; losing one's head may make it more difficult to transition smoothly into a Force ghost.
They pulled this kid out of slavery, away from his mother and everyone he knew, and dropped him in an unfamiliar setting on a completely different planet. He was a kid. The Jedi, with the exception of Obi Wan (and Qui Gon before he died), openly distrusted and even disliked Anakin.
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.
He regretted it the moment he did it. He asked himself what he just did, dropped his lightsaber, and was visibly ashamed of himself when Palpatine anointed him a Sith Lord and began his speech about wiping out the Jedi.