When Darth Vader locked sabers with Luke Skywalker in Return of the Jedi, it ended up perfectly flipping the outcome of Anakin’s prequel duel with Count Dooku in Star Wars: Episode III - Revenge of the Sith. The iconic Return of the Jedi duel between father and son starts with Emperor Palpatine attempting to seduce the young Jedi into retrieving his saber and striking him down. While it looks like Vader is trying to prove his loyalty to the Dark Lord by deflecting Luke’s saber, his actions actually hail back to a time when he faced a different opponent — one that ultimately furthered his own journey toward the Dark Side.
On a mission to rescue the kidnapped Palpatine in Revenge of the Sith, Anakin engages in a rematch with Count Dooku. The Jedi quickly triumphs over the Sith, cutting off his hands and pinning his head between two lightsabers, only to be coaxed by Palpatine into killing the defenseless Dooku. In Return of the Jedi, Palpatine tries to tempt Luke into taking back his lightsaber and striking him down, but Vader stops him from doing so.
Being lured to kill Count Dooku by Palpatine was a pivotal moment for Anakin, as even the troubled Jedi admitted that Dooku should be apprehended and face trial for his crimes. When Vader sees Palpatine trying to do the same thing to his son, he intervenes. Vader stopped Luke from killing the Emperor so that his son wouldn’t set out on a journey towards the Dark Side in the same way he did.
Anakin was never truly evil prior to committing himself to Palpatine's cause in Revenge of the Sith, but a series of traumatic events and a system that failed him led to his becoming Darth Vader. As Luke says in Return of the Jedi, “There is still good” in his father. When Palpatine told Anakin to kill Count Dooku, he was clearly conflicted, knowing that the act would in all likelihood be a direct violation of the Jedi code. When Anakin does kill the Count, he immediately regrets it, but Palpatine knows that this is just another step in his plan to fully convert the young Jedi to the Dark Side. In Return of the Jedi, it appears as if Vader is trying to protect the Emperor from being cut down by Luke, but in actuality, he is flipping the outcome of his son’s actions so that he does not make the same mistake he did when he killed Count Dooku.
In the end, it is Darth Vader who betrays Emperor Palpatine. In one last act of love, Vader spares his son in Star Wars: Return of the Jedi from doing the same terrible thing he did in Star Wars: Revenge of the Sith and instead takes his own step toward redemption by attempting to kill his former master. Count Dooku has served the franchise as a cautionary tale of what happens when a Jedi gives into temptation. Fortunately, Vader’s duel with Luke prevented the Jedi from doing just that.
