By the end of Return of the Jedi Luke has already killed thousands of Imperial troops by blowing up the first Death Star, sliced and diced his way through Jabba's pleasure barge not to mention shish-kebabbing and blastering literally dozens of random storm troopers.
My question is why would killing Darth Vader or The Emperor be the trigger to turn Luke to the Dark Side?
It isn't the act of impersonal killing, the kind of killing a soldier might do in the line of duty. Blowing up the Death Star while potentially horrible, Luke, as a Light Side (soon to be) Jedi, takes no pleasure in its destruction.
It was the very personal act of killing done at the bidding of Dark Jedi that helps turn a Jedi into a potential Sith-in-training.
This is why the Emperor goaded Anakin when he attacked him to tap into the power of the Force, but he enrages him while doing so, ensuring he taps the Dark Side of the Force due to his emotional state.
The same thing is done by Palpatine (and Vader) when Luke is fighting Vader in order to cause his rage to grow, to establish a feedback loop to the Dark Side.
If a Jedi grows used to using his rage as part of his connection to the Force, he can be seduced to the Dark Side slowly all the while believing he is simply growing more powerful in his use of the Force. Note how enraged Luke is while he is laying that glorious beat-down on Vader after the threat of turning Leia to the Dark Side.
"Your hate has made you powerful" said Palpatine as Luke stands over the defeated Vader.
My understanding is that it's not the killing that matters so much.
Each of Vader and the Emperor was plotting to overthrow the other, and each was fully aware of what the other was up to. Neither was strong enough on their own (Vader because he's damaged goods, the Emperor because he's older than in the Prequels), or neither felt confident of victory on their own, so they needed help: that's where Luke comes in. Luke is being asked to pick a side: join Vader and overthrow the Emperor, or join the Emperor and overthrow Vader.
Based on this reasoning, the assumption is that Luke has already turned before he picks a side and kills one. He's already serving the will of his new Master. The actual turning point was not the killing, it was Luke giving in to hatred on account of the Emperor's taunting about the (he thought) failed Rebel assault, or Vader's taunting about his sister.
The evidence for this reasoning firstly comes from Vader's conversation with the Emperor in Episode V (although it will become stronger later), and from the Rule of Two (which admittedly didn't exist when the Original Trilogy was made, but as we'll see later on, it didn't need to). Let's look at the conversation:
EMPEROR: The Force is strong with him. The son of Skywalker must not become a Jedi.
VADER: If he could be turned, he would become a powerful ally.
EMPEROR: Yes. Yes. He would be a great asset. Can it be done?
The important thing about this is that according to the Rule of Two, this cannot be done. Luke cannot join both of them: the Rule of Two doesn't allow it. So one of them has to go.
Furthermore, both of them would be aware of this. So even at this point, both of them have already accepted that if Luke turns, one of them is toast. From then on it's only a matter of which side he picks, at least so far as they're concerned.
Let's finish up with two more supporting quotes; first from Vader in Episode 5:
You can destroy the Emperor. He has foreseen this. It is your destiny. Join me, and together we can rule the galaxy as father and son.
And secondly from Palpatine in Episode 6, crucially as Luke seems about to kill Vader:
Good! Your hate has made you powerful. Now, fulfill your destiny and take your father's place at my side!
That seems the final conclusive proof. There's no need to invoke the EU, no need to go beyond the Original Trilogy, everything is there in those two quotes.
Each of Vader and the Emperor was plotting to overthrow the other [...] Neither was strong enough on their own -- this is not true. The Emperor was in charge the whole time and could have dispatched Vader if he wanted to. But Vader was his primary tool in both enforcing his will and gaining/turning Luke, so he was not about to discard him before getting Luke as his new apprentice.
– Matthew Read
Apr 29 '15 at 20:47
First lets think about how Anakin fell to the dark side.
He was desperate to save his wife, in his attempts to keep Sidious alive (and use his knowledge) he'd betrayed the jedi order and killed Windu. Sidious had been his friend since he'd arrived on Coruscant, he'd confided in him in the past (after the incident on Tatooine and when he'd killed Dooku). He'd passed the point of no return with the jedi, he felt they were holding him back the his only option was to embrace the dark side in an attempt to save Padme. The final straw was when Padme abandoned/betrayed him (and brought Obi Wan to kill him).
Lets move forward to the throne room.
Luke believed in the rebellion, he also believed Vader could be redeemed.
I don't believe the Emperor ever planned to keep two apprentices. Vader was far less powerful after his surgery, he wanted to replace him with Luke who was beginning to show real potential. To do this he created a trap.
He leaked false information to lure the rebellion into an all out attack. He intended to crush Luke's resolve in one swift stoke by destroying the rebellion in front of his eyes and forcing him to murder his own father.
If all had gone to plan then Luke would have been left with nothing, the rebellion would have been crushed, his friends would have been dead and he would have been responsible for his father's death (or Vader would kill/corrupt Leia). What options would he have? He'd have had Sidious' offer - join him and take control of the empire...
Of course things didn't exactly go to plan for Sidious!
In fact the original script planned to turn Luke to the Dark side. It was planned that he puts on Darths helmet and continues his "work". You can find a reference to this "fact" in an article on IO9.
Even though Lucas really wanted a bright, upbeat ending, and he fought against killing off any major characters — even Yoda, for a long time — during one story session, Lucas pitched a really, really dark ending. In a nutshell, the scene with Vader and the Emperor unspools the way it does in the final film. Vader sacrifices himself to take out the Emperor, and then Luke helps Vader to take off his famous helmet. And then — Luke puts on Vader's helmet himself. In the transcript of the story session with Lucas and Kasdan, Lucas says: "Luke takes his mask off. The mask is the very last thing — and then Luke puts it on and says, 'Now I am Vader.' Surprise! The ultimate twist. 'Now I will go and kill the [Rebel] fleet and I will rule the universe.'" Kasdan immediately responded, "That's what I think should happen" — but Lucas didn't actually want to go that dark because "this is for kids."
From The Empire Strikes Back:
YODA
Run! Yes. A Jedi's strength flows from the Force. But beware of the dark side. Anger... fear... aggression. The dark side of the Force are they.
Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice.LUKE
Vader. Is the dark side stronger?YODA
No... no... no. Quicker, easier, more seductive.LUKE
But how am I to know the good side from the bad?YODA
You will know. When you are calm, at peace. Passive. A Jedi uses the Force for knowledge and defense, never for attack.LUKE
But tell me why I can't...YODA
(interrupting)
No, no, there is no why. Nothing more will I teach you today.
Clear your mind of questions. Mmm. Mmmmmmmm.
Yoda attempts to impress upon Luke that the Jedi way is to act defensively, and never let emotions take over and rush into an attack. Doing so is a step towards the Dark Side of The Force.
The Emperor and Vader know this. By goading Luke into allowing his emotions and anger get the better of him, they intend to lure him to the other side. Instead, Luke chooses the way of the Jedi, and refuses to kill his father - saving him (for now) from stepping down the road to The Dark Side of the Force.
I think the one consistent thing I've noticed in the movies (specifically in Attack of the Clones and in Empire Strikes Back) is the notion that in order to defeat the Dark Side, you have to join the Dark Side. Both Dooku and Vader try to entice Obi-Wan and Luke, respectively, to the Dark Side.
It goes to show you the inconsistent logic that Lucas tries to portray the Dark Side (In order to defeat it you must join it). The only way it can possibly work is your emotions overwhelm your reasoning as well as a major incentive of "the ends justify the means" approach to resolve the conflict.
I personally don't think the Jedi are any better, which may or may not be Lucas' intention. The political undertones of democracy and dictatorship is simply a matter of degree, not a distinction in principle.
Luke was put into a position where he was forced to be hateful and see how much the Dark Side could actually benefit whatever Luke thought was good. Remember, Palpatine with Anakin said, that Good was a point of view. Anakin's "good" was love of his mother, and Amadala his wife.
Thats why its interesting what actually gets Luke to almost join the Dark Side. Luke was essentially about to be killed by Darth Vader, but then, Darth Vader puts him in the position to get angry, and says, "you have a sister... if you don't join the Dark Side... then perhaps she will..." Luke used the Dark side to defeat his father and prevent a future where his sister was being tested. This was supposed to be where he joins. This is because he realizes (or at least he should think) that the Dark Side could be used as a tool for good.
Its in that moment he throws his lightsaber away and shows what we have seen his father not do. It is actually what many of the Jedi Masters have said, which is to control your feelings and accept things that cannot be changed. However, it is seen, especially with Anakin, that they could get the power to control their destinies.
Killing Vader or the Emperor was supposed to be an end of a certain evil. It was almost as though it could be perceived as destroying one evil and joining the Dark Side to represent his own image of good. Which of course would be perverted by itself and become evil. In Darth Vader and the Emperor we see either the evil weapon or the evil leader. It was supposed to give this view that he could replace one of those and put in whatever he thought was good. In this case, he was more called to by his father which was the weapon. Which essentially gave this idea that as long as the Emperor was dead, the weapon could be used as a tool for good. Contrary, the Emperor made this image of his father being the evil one, and that so many of his good friends were being killed. So essentially, by replacing his father as the weapon, he would be able to control what happens to people. In the end though, Luke realizes that they are all one in the same, and become perverted by the Dark Side.
Giving in to hate and fear would turn Luke to the Dark Side. Given that Darth Vader is Luke’s father, Luke would have to hate him a lot to kill him. If, instead, he chooses to forgive him, and spare his life, that’s the light side.
A force sensitive being is corrupted by lusting for power, control and dominance. Killing out of fear, anger or other selfish motives is a path to the dark side.
Whereas destroying the deathstar was for the purpose of defending his friends and other innocent free people, and therefore light-sided, destroying the emperor to take over the galaxy would be Dark-sided, as would killing Vader in revenge.
Even if done with the best intentions, it is still a dark sided act. Mercy is a light-sided act.
Selfless, not selfish, that's the difference between light and dark, between evil and good.
Luke was in a no win situation. He wasn't powerful enough to defeat either Vader or The Emperor. He had to choose the lesser of two evils. When The Emperor said
Fulfill your destiny and take your father's place at my side!
he was telling Luke to embrace the power of the Dark-Side, kill his father and become the new apprentice because there won't be a three Sith Lord Empire - only two. Had Luke killed Vader, he'd have become Palpatine's apprentice or been killed with Sith Lightning.
Vader was more powerful than Luke and could have defeated him. But Vader wanted Luke as his own apprentice and Vader's own darkness had been compromised (by the first duel with Luke on Bespin). Vader was also the "Chosen One" and the only person who could defeat The Emperor and fulfill the prophesy. Emperor Palpatine and Darth Vader relied on their Sith Clairvoyance to plan Luke's seduction. They both saw Luke as a threat to destroy The Emperor. Neither man realized that what they were really seeing was Luke turning Vader back into the "Chosen One" through compassion to fulfill the prophesy (difficult to see - always in motion is the future).
So the act of killing Vader wouldn't have turned Luke to the Dark-Side... Luke being in a no-win situation would've had to choose the Dark-Side to stay alive then kill Vader to become the apprentice, or help Vader kill The Emperor and become the apprentice.
Luke chose to die. Vader screwed that up for him.
It doesn’t.
Yoda’s original plan for Luke was to conquer both Palpatine and Vader, as explained in Empire Strikes Back:
YODA Stopped they must be. On this all depends. Only a fully trained Jedi Knight with the Force as his ally will conquer Vader and his Emperor. If you end your training now, if you choose the quick and easy path, as Vader did, you will become an agent of evil.
Admittedly, Yoda is not explicit in his direction to Luke that the way to stop or conquer Vader and the Emperor involves homicide, so we cannot know what was unspoken within his mind.
