Ok but Empire Strikes Back and Revenge of the Sith are two of the most loved Star Wars movies in the community and they’re the most dark and depressing ones.
As I said, even the darkest Star Wars movies still follow heroes’ perspectives and show them acting heroically, and the movies end on a hopeful note. A Vader movie would be only darkness and angst all the way through, with the only outcome being that more Jedi would be killed by the end, and perhaps Vader would be more powerful.