Even back in the original Star Wars, Ben appears to have zero hope for redeeming Vader. “Only a master of evil, Darth.” The problem with Vader’s ROTJ line is that it’s a reference to a hypothetical prequel story that will never exist. The Obi-Wan show had the opportunity to right this wrong, but instead Ben just expressed how sorry he was and nothing beyond that.

I think the idea that the Jedi dogma is to destroy evil instead of redeem it is what makes the entire OT and sequels so good. Luke is doing what his mentors would never do with Vader, blazing a new path forward. It pays off in the end, but he instinctively relapses toward his old teachings with Ben Solo, thus why he feels the Jedi need to end.