But they did turn his redemption arc of killing Palpatine into nothing by bringing Palpatine back jsut for the sake of Kylo Ren not being a villain.
Palpatine never came back. His character was simply the embodiment of all Sith and the dark side. And it's not like we don't have entire religions in real life based around people being the emdodiment of good and returning to life after death. Personally, I think that Rey being a part of the dark side is such a great antithesis to the story of Vader. It's quite beautiful to show how someone can fall from the dark side as well.