After defeating Maul and killing his brother Savage Opress, Sidious takes him to a Separatist prison, where Count Dooku tortures him for information about the Shadow Collective. Prime Minister Almec arranges Maul's escape and the latter then heads back to Zanbar to command the Death Watch army.
Though he killed Jinn, Maul was, in turn, defeated by Kenobi, who bisected the Sith assassin at the waist. Darth Maul was believed to be dead in the aftermath of the battle but ultimately survived, emerging a decade after his defeat, during the height of the Clone Wars.
As they approached Maul's cell, Kast and Saxon took out the droidekas with a rocket, destroying them and clearing the approach to the cell. Once they had Maul, they freed him from his restraints and informed him that they had arrived to rescue him.
Thought dead, Darth Maul survived his injuries by focusing on his hatred of Obi-Wan Kenobi, the Jedi who cut him in half. His shattered body was dumped amid the refuse of the junk planet Lotho Minor, where the once deadly warrior fell into madness, staying alive on a diet of vermin.
Palpatine Never Thought Highly Of Darth Maul as an Apprentice. To clarify, Palpatine still considered Maul to be a piece on the galactic board and used him accordingly, namely to bait Mother Talzin into battle and to manipulate him into disposing of other underlings.
What happened to Maul after Palpatine captured him?
After being captured by his former master on Mandalore, Maul is imprisoned in the Spire on Stygeon Prime. Sidious and Count Dooku allow Maul to escape, hoping that he will lead them to Mother Talzin, leader of the Nightsisters, so she can be destroyed.
Though seemingly killed by Obi-Wan Kenobi at the end of the film, Darth Maul returned in the 2008 animated series Star Wars: The Clone Wars (voiced by Sam Witwer).
In the now-abandoned Legends continuity, his birth name appeared to be Maul, with the title of "Darth" simply added to it when he was taken on as an apprentice by Darth Sidious.
He regretted it because he had essentially wasted 20 years training him. But since he was an enemy at the time of his death on Tatooine, Sid was glad that he did not have to keep watching his back anymore now that the loose ends were tied off.
The two were able to free Maul after he had been interrogated by the Sith Lord Count Dooku, and together the three rendezvoused with more Death Watch forces on the moon of Zanbar. Maul took command of his forces once again, and assured them that the war they had longed for was at hand.
It's already been shown in the source material… more than once… that Darth Maul is not on Darth Sidious's level. Every time he dueled Sidious as his apprentice (in Legends) he got easily defeated.
Transference. Sidious used Plagueis' methods to transfer his consciousness to a new body after his first demise. During the Battle of Endor in 4 ABY, Sidious was killed by a redeemed Anakin Skywalker, who turned back to the light side of the Force and threw Sidious down a reactor shaft to save his son Luke Skywalker.
After tracking Kenobi to Tatooine, Maul engaged him in a brief Lightsaber duel, which resulted in Maul's demise. Before dying, Maul was comforted with the knowledge that Kenobi was protecting the Chosen One, finding some peace in the idea that the end of the Sith was at hand and that he would be avenged.
Maul's skills, on the other hand, became stagnant as he spent the last few decades of his life keeping himself alive and plotting schemes. Given this, it makes sense Obi-Wan quickly defeated Maul in their final duel in Star Wars Rebels.
Regardless, based on the season 7 of The Clone Wars, and the fact that Darth Maul was aware of Anakin Skywalker's future and his role as the Chosen One, it is very likely that Darth Maul indeed feared Darth Vader to some extent, and preferred to avoid a direct confrontation with him, at least, on his own.
Maul most likely knew Sidious' double role as Palpatine and Sidious when he served as Palpatine's apprentice. He definitely knew about Palpatine's goal of creating a galactic empire ruled by the Sith. Maul knows that Sidious is the Emperor.
Overall, Dooku was far more equipped to become a Sith Master than Maul was. Had he managed to overthrow Palpatine, Dooku would have been able to use his position, combat abilities, and dark side Force powers to take his place on the throne Palpatine had carved.
Indeed he did. Dooku (aka Darth Tyrannus) knew his master's identity. At this point the Jedi thought Dooku was the Sith Master. The kidnapping was planned to reinforce public support for Palpatine while further confusing the Jedi about his true identity.
Palpatine despised Darth Maul for not being "quite good enough" and lost interest in him once his gaze turned towards Anakin Skywalker. Palpatine saw Maul as a pawn and used him to his advantage, but never considered him a direct threat to his life.
In this scenario, Maul would cause Luke Skywalker to fall to the dark side and then begin training him as an unofficial Sith. With Obi-Wan Kenobi dead, Maul's thirst for revenge wouldn't end, and he'd set his sights on a more dangerous opponent.
Even though Darth Plagueis knew of Maul's existence, he approved Palpatine training him since Maul was always meant to be cast aside. Nevertheless, Palpatine was setting Darth Maul up as his apprentice-to-be, knowing he would eventually kill Plagueis and take the Sith Master title for himself.
