Just suppose you've never had the story told to you, not the version that everyone knows. Suppose you've heard it told in a way where Darth Vader and Obi-Wan Kenobi never share the same room. What might you begin to believe about them? What might the relationship between them begin to look like, from a certain point of view?
Just suppose this: Darth Vader is Obi-Wan Kenobi.
After the fateful clash between Kenobi and Anakin Skywalker at Mustafar, only one leaves the planet of his own accord. But what if only one truly leaves the planet at all? Kenobi finishes the job there and then, ending the life of the rebellious Jedi and would-be Sith apprentice Skywalker, but then can no longer live with himself, having murdered his long-time friend and ally, and so he replaces him as apprentice to Palpatine, ruler of the Empire and Darth Sidious, keeper of the Sith flame.
Except he can't let the galaxy know what he's done. Even in the version you know, Kenobi goes into anonymous exile. Certainly, Yoda does the same, but Kenobi is the one who can't fully remove himself from matters of the Force, one way or another, remaining active, whether in guiding the young son of Skywalker to maturity, or in becoming the watchdog of the Empire, the last remnant of Force lore active in the galaxy.
As Vader, even if he operates as a Sith, Kenobi has all the opportunity he needs to correct his mistake. Taking the life of Skywalker ended the Jedi Order's chances of influencing Palpatine's further movements. To keep hope alive, Kenobi must replace him. It's a life of constant danger. If Palpatine suspects the truth, the gambit is finished in an instant. To everyone else Vader is invincible, but not to a master of the Sith. Even if he's not compromised, fused with metal parts the way Skywalker needs to be in the version you know, Kenobi never had full command of his abilities, the way Yoda did, or Palpatine. He would be overmatched, without a way to expand his powers, no guidance possible and no time to himself to learn on his own, always carrying out the bidding of the Empire or Palpatine directly. He seems autonomous but he isn't.
It's a mission with a long-term goal, waiting for an opportunity, no matter how long it takes. Either way he must recruit Skywalker's son, who is instantly a wildcard, someone Palpatine can't control. As Vader, Kenobi can manipulate the truth with impunity. First, the boy won't trust him. But when he learns the truth he will do anything to live up to the hope he represents.
But it is a costly life, a costly lie, for Kenobi. He will be scorned by those he once loved and distrusted even by his allies. It will be lonely. You see, there are so few differences between what you know and what might have been, what's to quibble? Kenobi might as well have been Vader all along.
It might even have been easier this way.