system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Why was Vader protecting the Emperor in the Return of the Jedi?

In the famous scene in Episode V, when Vader cuts Luke's hand, he tells him they can destroy the Emperor together and rule the galaxy as father and son.

However, in Episode VI, when Luke hears enough from the Emperor and pulls his lightsaber to kill him, he is blocked in the last second by Vader. 

Why would he do that?
gpt
Vader was in a constant struggle between light and dark. On one hand he wanted Luke to join him on the dark side but at the same time he wanted to protect Luke, as any good father would want for his son, and prevent him from falling. If Luke had killed Palpatine at that moment he would have fallen to the dark side. In addition to that, the dark side of Vader was still the "apprentice" of Palpatine and as such would protect his master. So in that moment stopping Luke actually managed to satisfy both sides of Vader.