Depends on how you want to tell the story. Those of us watching the prequel trilogy knew that Senator Palpatine was going to be the Emperor all along, and that Anakin Skywalker was going to be Darth Vader. The point of telling the story, and us watching it, was to see how they got there. Quite a few of us also knew about the Red Wedding in Game of Thrones before it was revealed in all its gory and it was still interesting to watch.
Not every treacherous plot needs to be a plot twist surprise reveal for the audience (even if your name is M. Night Shyamalan, hmmm, maybe especially if your name is M. Night Shyamalan).