system
Given the following post, respond as if you're commenting on Reddit:
human
[Star wars] Why didn't Vader overthrow the emperor earlier and take control of the empire?
gpt
The Rule of Two states that there must be two Sith, a master and an apprentice. It is the Master's job to hold power, and the apprentice's job to crave it. Emperor Palpatine always knew that Vader would one day try to overthrow him, in fact, that is the very essence of the relationship between a Sith Master and his apprentice. Both parties knew what was up.

Vader didn't move before the events of episode VI because he didn't think he could take out the more powerful Darth Sidious (Emperor Palpatine). When his son showed up, Vader recognized his power and saw a way to take out Sidious with the help of Luke. Before that, Vader wasn't sure he would win, and the consequences of failure could have been drastic.