Warning! SPOILERS for Obi-Wan Kenobi episode 5.
Anakin's comments to his master in Disney+ show and the Star Wars prequels. Part of this connection is Anakin's prediction of how Obi-Wan would fail to defeat his enemy in their lightsaber duel on Mustafar.
Obi-Wan Kenobi episode 5 has Anakin Skywalker and Obi-Wan Kenobi sparring in the Jedi temple on Coruscant. The scene takes place around the time of Star Wars: Episode II: Attack of the Clones when Anakin was still a padawan, before his turn to the Dark Side in Revenge of the Sith. Even during their friendly sparring matches, Obi-Wan witnesses Anakin’s focus on strength and power tip over into hostile aggression. Criticizing Anakin in his attacks, Obi-Wan says that "a Jedi's goal is to defend life, not take it." As they turn back-to-back Anakin retorts, "Mercy doesn't defeat an enemy master... That's why you're going to lose!"
Anakin's suggestion is that Obi-Wan's comion for life is a weakness, a mistake in the heat of battle that can only lead to defeat. Without knowing it, Anakin has predicted the mistake that Obi-Wan eventually makes in Revenge of the Sith when he fails to kill Anakin following their duel. Instead, leaving him to die on the banks of a lava flow is how Anakin survives and ultimately transforms into Darth Vader. Obi-Wan's act of mercy is a mistake that dooms countless innocents to the evil Darth Vader soon thereafter unleashes upon the galaxy.
Why Obi-Wan Kenobi Didn't Kill Anakin In Revenge Of The Sith
Anakin and Obi-Wan have always had a complicated relationship. Though they are friends up until Revenge of the Sith, they also have a mentor-mentee Jedi Master and Padawan relationship that links them to the great multi-generational chain of the Jedi tradition. Anakin sees Obi-Wan as both a mentor and a father figure. All these different and conflicting relationships converge once Anakin turns to the Dark Side. While Darth Vader has no hesitation in killing him, Obi-Wan Kenobi still sees his student and friend Anakin in Darth Vader. Though he is the first to draw his lightsaber, ultimately Obi-Wan cannot bring himself to kill Anakin on the lava shores of Mustafar. His love for Anakin is his mistake. In the moment he believes he has defeated Anakin, and that he will succumb to his wounds. First and foremost a Jedi, Obi-Wan doesn't take life lightly, even if he believes that it is already lost.
Until the events of the Disney+ show, A New Hope and captured his former master rather than striking him down, Obi-Wan wouldn't have become one with the Force. He wouldn't have guided Luke in the Battle of Yavin or sent him to Yoda in The Empire Strikes Back, all of which ultimately led to Anakin Skywalker's redemption in Return of the Jedi. The lesson of defending life in Obi-Wan Kenobi episode 5 might have been lost on Anakin, but in the end Obi-Wan’s mercy on Mustafar is ultimately what defeated the enemy.