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.