Darth Vader waited until the Return of the Jedi to turn on Emperor Palpatine — why? The Empire Strikes Back pulled arguably one of the best plot twists in cinemas — the reveal that Darth Vader, the main antagonist of the original movie, was the father of its hero, Luke Skywalker. This became a central plot point moving forward resulting in the Sith Master eventually betraying Darth Sidious to save his son. While it offered a satisfying ending, it also posed some questions regarding how Anakin Skywalker turned to the dark side.

This was where the idea for the Star Wars prequel trilogy came about. As convoluted as those films can be, it effectively enriched the lore with a focus on how the Jedi, the Sith, and the galaxy worked prior to the rise of the evil Empire. More importantly, it also traced Darth Vader's origins as a boy from Tatooine who grew up to be a powerful Jedi, only to be corrupted by Emperor Palpatine.

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The prequels functioned as a great supplementary material to complete Darth Vader's story — chronicling the beginning of his journey to be a Jedi, his fall to the dark side, and redemption in the Return of the Jedi. That said, it's still curious why it took him so long to turn on Emperor Palpatine — not only did Emperor Palpatine fail to fulfil his promise of keeping Padmé Amidala alive, he also played a part in her death, and keeping his lover alive was Darth Vader's only goal. Without that, Emperor Palpatine really didn't have a bargaining chip against Anakin, so it's curious why it took him so long to make his move to defeat the Sith Master. Given the physical and psychological trauma he had gone through since Star Wars: Episode III - Revenge of the Sith, Darth Vader wasn't confident enough that he can succeed if he antagonized Emperor Palpatine. Things made a significant turn, however, with Luke's emergence, prompting him to finally push through with his scheme even before the events of Return of the Jedi.

Darth Vader and Luke Skywalker in Star Wars The Empire Strikes Back

In The Empire Strikes Back, he created a shadow Empire in preparation for overthrowing Emperor Palpatine after he found out that Luke was his son. His people helped him hunt down the young Skywalker in the hopes that he would join him in his plan to defeat Darth Sidious. As Darth Vader told Luke at the end of the said film, he "can destroy the Emperor," and if he teams up with him, together, they "can rule the galaxy as father and son." So, while Darth Vader's intentions at this point weren't good since the only reason he wanted to defeat Darth Sidious was so he could take his place as the ruler of the galaxy, he did have intentions of betraying. Considering that he put enormous effort into locating Luke and risked not killing him right away, chances are that he had been tinkering about his plot against Emperor Palpatine for a considerable amount of time.

In Star Wars: Return of the Jedi, Darth Vader felt defeated and more importantly trapped, again. He was still reeling over Luke's rejection of his offer and felt that at this juncture, he was far too gone to the dark side, so he might as well remained loyal to Emperor Palpatine. Seeing Luke's selflessness, however, in the movie's final confrontation inspired him to finally do the right thing, save his son, and defeat Darth Sidious — even if that meant sacrificing his life in the process.

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