Anakin as Darth Vader in ROTS is more of a serious threat than he is in ROTJ. I'm not talking about him in the iconic Vader suit either, just the newly christened, robed Anakin Darth Vader.
In ROTS, Vader is being the Emperor's little ***** because he doesn't want his family to die and feels betrayed by his friends. With a battalion of troops he goes and masacares a bunch of said Jedi friends and children, goes to Mustafar and murders the surrending Separtatists and gets his limbs lobbed off while burning alive in agony. Pretty hardcore stuff.
In ROTJ, Vader is being the Emperor's little ***** . . . for absolutely no reason at all. He just "must obey his master". All he does during the movie is pick on Moff Jerjerrod and tries to
poorly convince Luke to join the emperor (nothing he says is convincing). He's belittled, talked down to, gets his ass handed to him by Luke and turns out to be an old marshmallow man. Then he's somehow forgiven for all of his sins. It didn't take the Blu Ray "noooo" to make him suck, he was always planned that way, even in deleted scenes.
Look at him tapping his fingers onto his chest like a goof and ceasing his actions because "it's what the Emperor commands".
He was willing to get himself killed so his son could take his place in a life of misery at his master's side. Doesn't get much more pathetic than that. This was the guy from Star Wars and Empire that hated the Rebels and had aspirations to overthrow the Emperor and rule the galaxy on his own.
Rogue One gave him his balls back for sure, but it wasn't the prequels that took them away. Anakin Skywalker sucked long before 1999.
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