Emperor Palpatine thought Darth Vader's death and simultaneous betrayal in Return of the Jedi was pointless. Anakin Skywalker fully turned to the dark side of the Force in Star Wars: Episode III - Revenge of the Sith, but like his grandson, Ben Solo, he felt the pull back to the light. As Darth Vader says in A New Hope, Obi-Wan Kenobi attempted to save him from his mistakes and lure the Anakin part of him back out, but only Luke Skywalker was successful.
Luke fought and ultimately turned Darth Vader back to the light towards the end of Return of the Jedi. Instead of killing his son and securing his future by Palpatine's side, Vader chose to pick up Palpatine and toss him down the Death Star's shaft. The thing is, Palpatine never really died, since he returned years later in Star Wars: The Rise of Skywalker in a clone body. And during his time on Exegol, Palpatine recorded his life as a Dark Lord of the Sith as well as the origin and future of the Sith Order in the book Secrets of the Sith, in which he also discussed Vader's death.
According to Palpatine, by killing Obi-Wan Kenobi aboard the first Death Star, Darth Vader redeemed himself after he failed to defeat his former master on Mutafar. In the aftermath of that duel and Padme's subsequent death, Vader's life was powered by "fear, pain, and suffering," but when he found out Luke was alive, he began to feel love again. It was for that love that Vader killed Palpatine, but according to Darth Sidious himself, it was a "pathetic display" and "all for nothing," because in the end, Darth Vader sacrificed his own life yet "the Sith would return."
In a way, Palpatine is right; Vader's death only delayed the inevitable and despite being the Chosen One, Anakin never achieved true balance in the Force. His destruction of both the Jedi and Sith Orders was only temporary; it was ultimately Rey who found a way to bring balance to the Force and begin anew, but only after the First Order terrorized the galaxy and the Sith returned in The Rise of Skywalker. But there's also another perspective here, as is always the case with Star Wars.
Palpatine may have been right on one hand when it came to Vader's death, he was wrong overall, because Vader's sacrifice meant saving Luke Skywalker's life. Without Luke, the First Order, Final Order, and Palpatine's defeat all may not have been possible. He was the impetus for virtually everything, while his sister, Leia Organa, was the final piece in the puzzle, training Rey and leading the Resistance.