Emperor Palpatine should have known Darth Vader would betray him in Return of the Jedi, but he was blinded by his arrogance and ignorance. Palpatine couldn't predict Darth Vader's betrayal in Return of the Jedi for one simple, yet very powerful reason.
Never. Palpatine never cared for Anakin; he always saw him as a tool, albeit an extremely useful one, and manipulated him constantly to take advantage of his power.
A Master without an apprentice is a Master of nothing. So he does see Vader as a tool, but he sees everyone as a tool. There's a scene in the Bounty Hunter Wars trilogy where he outright calls Vader a tool and Vader agrees with it: The Emperor nodded, mulling over Xizor's words.
Despite Vader's seeming loyalty to the Emperor, Palpatine wanted to replace his servant with a new apprentice, while Vader plotted to overthrow the Emperor so he could rule the galaxy with his son. In the end, Luke surrendered himself to Vader on the forest moon of Endor.
He just didn't have the allies to make it happen, explaining why he had relied on Luke to join him. Taking after his master, Vader evidently learned from the tragedy of Darth Plagueis the wise. Unfortunately, he just lacked the proper resources.
EXTRA footage from 1977 REVEALS Darth Vader’s Backstory
Who did Palpatine fear?
As powerful as Darth Sidious may have been, it's easy to forget he actually seemed afraid of Master Yoda. As seen in Revenge of the Sith, Palpatine initially thought he could deal with the aged Jedi Master - and then, when Yoda proved able to counter the Sith Lord's Force lightning, he fled.
Vader was the one doing the dirty work on the ground, whether it be killing Rebels by the score with his lightsaber, his fighter, or his fleet. Palpatine may have given the orders but Vader was the one doing the killing, personally depriving families of their loved ones and doing it with relish.
Although the decision to turn to the Dark Side was Anakin's alone, the choice did not come from thin air. His natural prone to impulsivity and emotion without the support systems he needed in place made it easier for Anakin to stumble down a path of darkness.
The short answer is yes, without a doubt, Vader regretted becoming a cybernetic monstrosity. First, there's the constant pain and discomfort from his suit. Worse, however, is the psychological pain. Vader destroyed everything he had ever known for a chance to save Padme.
Darth Vader couldn't kill Palpatine before Return of the Jedi because he was everything to him, as that's how consumed with hatred Darth Vader was. It wasn't until he allowed himself to be filled with love for his son, and look past Palpatine at something more important, that Vader was able to finally kill his master.
Vader's one friend he trusted the most was Erv Lekauf, the Dark Lord's personal adjutant. After he was heavily injured in a fire, Lekauf was force to retire. Erv's bravery, honorable, responsible, personality gave Vader his respects and the two saved each others' lives a few times.
Palpatine's relationship with Anakin follows the pattern of grooming for abuse, and then finally just actual abuse from beginning to end. It started when he was 9 years old, and basically didn't end until he threw him down a reactor shaft.
Darth Vader doesn't fear the Emperor, he despises him, just as Vader so vehemently despises himself. So, if not Palpatine, who could Vader possibly fear? Well, the answer is deceptively simply, yet incredibly shocking all at the same time: Luke Skywalker.
Though Palpatine regarded Vader with the caution common in a Sith Lord to their apprentice, he nevertheless entrusted much of his plan to Vader's abilities. He even referred to the broken man as his friend on numerous occasions, though he still found himself betrayed by his apprentice during Return of the Jedi.
Summary. Yoda recognized that he couldn't defeat Palpatine in one-on-one combat, as Palpatine's power in the Force was greater, and continuing to fight would likely result in his demise.
However, when it comes to who Darth Vader hates more than anyone in the Star Wars galaxy, himself and Obi-Wan Kenobi don't hold a candle to the true point of Darth Vader's hatred, Emperor Palpatine.
Anakin does not have even the slightest bit of remorse. By the end, he feels nothing but anger. Although he feels regret for killing Padme, he still helps the evil Emperor. The film ends with Anakins standing side by side with the man on whose behalf he has committed all his crimes.
The canonical comic Star Wars: Darth Vader #7 confirms the younglings' deaths caused Anakin grief, self-loathing, hatred and pain, fueling his descent even further into the dark side.
What would have happened if Anakin didn't turn bad?
Anakin would have likely been killed by the clone troopers, along with the other Jedi. However, perhaps he would have escaped. He would have taken the pregnant Padme with him. He would have needed to get as far away from the eventual Empire as possible, so perhaps they would have gone with Yoda to Dagobah?
Kylo and Rey (Daisy Ridley) also blasted the Imperial Crusier open trying to Force pull the Lightsaber away from each other, so clearly, Kylo Ren is an extremely strong Force-wielder. Nonetheless, it can be said that Darth Vader is the more powerful master of the Force for one key reason: his temperament.
Yoda's presence on Dagobah was likely hidden due to his mastery of Force Stealth and the planet's connection to the Force. Yoda, with his enhanced Force abilities and potential home-field advantage, would have had the upper hand against Darth Vader if they engaged in a lightsaber duel.
