Vader was fine in Jedi. His change started at the end of Empire Strikes Back. His final shot, on the bridge of the Executor, when the Falcon escapes. His son escapes, and he stares longingly after him into the depths of space. He starts to turn, but then looks back, one more time, as in disbelief. When he walks off the bridge, the officers, particularly Piett, are fearful for their lives. And yet, he just quietly walks off the bridge.
You don’t need a villain to take off his mask and cry and show you his tender heart in order to have depth. In fact, that is the lazy, amateurish way to do it.
No, Vader’s moment of vulnerability was just quietly walking off the bridge, back to the camera. Off to console himself with his own emotions and thoughts in some private chamber somewhere else on the ship.
Now fast forward to Jedi and it’s been a couple years later. The Emperor knows that Vader is a traitor. The only reason he keeps Vader alive is because he knows Luke will come for him. Otherwise, he would just assume kill Vader already, and he keeps him on a short leash and mocks him repeatedly when they speak together in person. For all we know, The Emperor had S/M sessions and tortured Vader half to death off camera during the years in between movies. Or maybe Vader has softened because of his feelings towards his son. Who knows the entire story, but Vader is broken and conflicted by the last film.
I don’t see anything wrong it.