So why didnt Obi Wan take out Vader? Obviously he couldnt because of the original movies, but I dont understand why Obi Wan walked away? Because he was sad?
as I read it, Obi Wan felt guilty for years because he thought he had killed his friend Anakin on Mustaphar.
But, in the fight scene on the TV show, Darth/Anakin had the line "you didn't kill Anakin. I killed him."
meaning that Obi Wan wasn't responsible for Anakin becoming Vader. So Obi Wan could walk away with a clean conscience.
granted, he is leaving Darth alive, but he apparently made a decision that he was not going to be the one to kill Vader.
and yes, that is a huge issue with prequel stories. one way or another, they have to end in a way that matches up with what we already know about the future of those characters.