At the end of The Return of the Jedi, the emperor tells Luke just after Luke defeated Vader in a duel,

"Now fulfill your destiny and take your father's place at my side."

If I was Vader, I would be thinking, "Palpatine is just going to throw my life away like he did Count Dooku. I'm a goner unless I take out the emperor soon!"

Video snippet from The Return of the Jedi

The Emperor then turns his back on Darth Vader while zapping Luke with bolts of electricity. Palpatine should know not to trust Vader. He seems so engrossed in torturing Luke that he doesn't notice Vader walking up behind him and grabbing him.

He should know not to trust Vader for a second. He knows a Sith apprentice becomes a master by killing the previous master. He could have killed Luke quickly and then kill Vader, instead of spending almost two minutes torturing Luke. He also had a chance to zap Vader a couple of times when Vader was nearly helpless, lying there on the bridge before getting up to stand by his master's side. One zap to Vader's breather would probably have short-circuited it.

Why did he make himself vulnerable to Darth Vader?

Edit to address why this is not a duplicate of another question.

The other question focuses on whether Palpatine erred in torturing Luke in front of his father. Palpatine already suspects Vader's feelings for Luke.

Palpatine: Strange that I have not felt him. Are your feelings clear on this?

Vader: They are clear, my lord.

My question is about why Palpatine would trust Vader since he knows the Rule of Two. It is a Sith tradition for Sith Lords to kill their masters (or apprentices). He should know to never trust Vader for even half a second, yet he makes himself vulnerable for almost two minutes. That was all the time Vader needed to pick up the Emperor and throw him down the elevator shaft.

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    This question is very similar to this one, possibly a duplicate. – Null Dec 16 '16 at 15:23
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    @Null I don't see it as a duplicate. The link you provided is about whether Palpatine thought torturing a son in front of his father was a good idea since maybe Vader still had feelings for his son. My question is more about why Palpatine would trust Vader since he knows the rule is for Sith Lords to kill their masters (or apprentices). That has nothing to do with whether Vader would stand to see his son tortured and killed. It's more about why Palpatine made himself vulnerable for a moment around somebody he know would eventually kill him, as Sith apprentices are known to do. – RichS Dec 16 '16 at 17:51

That is best answered by using the Star Wars Force Unleashed game series. It's not exactly canon, but it provides a view of this relationship.

In the game, Vader knows that Palpatine is looking for a new apprentice. After all, Vader is just a shell of a sith that Anakin could have been. So he tries what every Sith tries. To overcome his master. Vader finds a suitable apprentice and tries to plot against the Palpatine. Long story short, he fails, and Emperor spares Vader's life in exchange of his unwavering loyalty.

Therefore, Anakin knows, that he has no chance of going against his master. He is a beaten dog in Palpatine's service. At that moment, he needs the power of the dark side, and the Emperor provides it to him. He is like an addict, who becomes a slave of his master. Even though he secretly hates him, he cannot dare to defy him and is unquestionably loyal to him. It is very "unhealthy relationship", but Vader does not have a choice. Such was the power of the dark side the Emperor used. Vader knows, that if Emperor chooses a new apprentice, he is done for, but he is so shackled by the fear and hunger for power, that he follows the Emperor orders and tries to seduce Luke to the Dark side, even if that meant that Emperor could choose Luke and get rid of Vader.

That is, until Luke shows Vader that he can still be saved. At that moment, he feels the Light side again, and the love for his son is stronger than the addiction and fear that Palpatine created.

  • The Force Unleashed games aren't canon, though. – Null Dec 16 '16 at 15:23
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    The answer is useful, but focuses more on why Vader is in the iron grip of Palpatine. It doesn't answer the question of why Palpatine would make himself vulnerable to Vader. Unless you want to say that because Palpatine over-estimated his grip on Vader, he did not suspect Vader would kill him. – RichS Dec 16 '16 at 18:00
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    @RichS yeah you're right, that's what i meant. Palpatine underestimated the power of the light side of Luke. – Sebustyan Dec 19 '16 at 7:56

Judging from the dialogues between Luke, Vader and the Emperor, I guess that the Emperor didn't necessarily trust Vader, but trusted in his own godlike powers to make plans work

  • "Soon the Rebellion will be crushed and young Skywalker will be one of us!";
  • "Everything that has transpired has done so according to my design.";
  • "From here you will witness the final destruction of the Alliance, and the end of your insignificant Rebellion.";
  • "It is unavoidable. It is your destiny. You, like your father, are now mine!"

and that Vader "can never be turned from the dark side." In the Emperor's mind the outcome of the confrontation he aims for is already reality. After a while he even starts to call Luke "my young apprentice".

As for Vader, it is not clear if the Emperor senses betrayal in Vader, at least everything Vader says on the landing platform and in the throne room indicates that he believes to be in the iron grip of the dark side and that he would rather kill Luke than turn from it

  • "You don't know the power of the dark side. I must obey my master.";
  • "If that is your destiny.";
  • "It is too late for me, son.";
  • "Give yourself to the dark side. It is the only way you can save your friends.";
  • "If you will not turn to the dark side, then perhaps she will."

The Emperor has no reason to believe that Vader has changed his attitude towards him. For all he knows Vader is still Vader. When the Emperor is in the middle of slowly killing Luke with his force bolts, he just doesn't sense Anakin winning over Vader, since he's so focused, and he doesn't sense Anakin attacking him, since he stands right beside him.

(All quotes are taken from this script)

  • That answers a different question. "Did Palpatine foresee Vader turning back to the Light Side of the Force?" My question is more about why Palpatine would trust Vader since he knows the rule is for Sith Lords to kill their masters (or apprentices). That has nothing to do with Vader would turn back to the Light Side. – RichS Dec 16 '16 at 17:47
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    @RichS: when the movie was made, there were no Sith, no rule of two and Darth wasn't a title but a name. I derived my answer from the dialogues and actions of the characters in the movie without taking the prequels and the expanded universe into consideration. – Marvin Falz Dec 16 '16 at 18:41

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