Darth Vader and Palpatine’s relationship in Star Wars is hardly something that prompts blind loyalty and unfaltering servitude, so why did the former wait so long to kill the Emperor?
Despite being a bright member of the Jedi Order, Anakin still fell for Darth Sidious’ plan and did his bidding to become Darth Vader. But as the character learns in Revenge of the Sith, not only does the Supreme Chancellor fail to live up to his promise and save Padme, but he also indirectly causes her death. Why, then, would he stick by the Emperor’s side after figuring out the truth? In fact, Vader remained a loyal servant until the very end and if it weren’t for Luke, he never would’ve done anything to jeopardize his master.
This is especially interesting since we know that the fallen Anakin from the last movie in the Prequel Trilogy developed the ambition to overthrow the Chancellor, but didn’t go through with it after his duel with Obi-Wan on Mustafar. So, what changed? Well, it appears that Star Wars has already answered this in a novel. In Dark Lord: The Rise of Vader, it’s explained that despite having the strength to kill his master, Vader wasn’t in the right psychological state to go through with it.
In fact, if Vader had managed to defeat and kill Obi-Wan, he would’ve done the same with Palpatine. But as we saw, the battle left him severely wounded, and the death of Padme made sure that he wasn’t in his right mind. Additionally, George Lucas once told us that Vader became weak after receiving all those injuries which made him less powerful than the Emperor and stripped him of his confidence. In his prime, Anakin could’ve easily defeated the sinister Sith Lord.
It wasn’t until the Emperor betrayed him again and almost killed his son that Vader found the strength to do what he should’ve done ages ago. Though in the interest of the greater scheme of things, Sidious survived his fall and returned to haunt the galaxy one more time in Disney’s Star Wars Sequel Trilogy.