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.
Sidious revealed to Maul that he did not intend to kill his former apprentice, as he still had use for Maul; it was Sidious' hope that Maul would lead the Sith to Mother Talzin, Maul's mother and the leader of the Nightsisters, so she could be eliminated as a potential threat to the eventual Sith rule of the galaxy.
In Star Wars lore, Emperor Palpatine took multiple apprentices before Darth Vader, and Darth Maul was among his most famous recruits. Maul, a fan-favorite character, gained a massive following after The Phantom Menace was released in theaters, but unlike his many fans, Palpatine despised Maul for many reasons.
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.
Why Palpatine HATED the Most Powerful Sith In all of Star Wars
What did Dooku think of Maul?
Dooku despises Maul for being a threat to him, and for killing his beloved padawan Qui-Gon, and Maul hated Dooku for being his replacement. They also didn't regard each other as a true Sith: Dooku saw Maul as nothing more than a trained animal and Maul saw Dooku as a Sith pretender.
For the majority of the battle, the future Emperor is seen cackling as he throws Maul and Savage around. He laughs at his victims as he's essentially toying with them. It further proves that his battle was all too easy for Darth Sidious as he's very much in a higher weight class compared to Maul and his brother.
Sensing his potential, Sidious took Maul as an apprentice while he was still a child in 40 BBY. Anointed by his Sith Master as Darth Maul, the Dathomirian was raised to hate the Jedi Knights that served the light side of the Force.
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.
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.
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.
Maul also never openly opposed Sidious. He became a rival in a way, but he knew he never really stood a chance of overthrowing him, specially after his injury. Palpatine had so much power and control over the senate.
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.
"Is it the Chosen One?" Maul asked. "He is," Kenobi replied. Maul looked up, the life fading from his eyes, and uttered his last words. "He will avenge us."
Yep. Darth Maul didn't really say or show straight up that he "feared" Darth Vader, but he openly stated his belief that he could not defeat Darth Vader without the help of the Jedi, therefore on his own.
And while his warnings don't work, at the very least, it shows that Maul became aware of Anakin and what he would soon become. With the rise of the Empire, it's likely Maul caught on to Vader's existence pretty quickly.
Towards the end of their relationship as Master and apprentice Palpatine began to resent Plagueis for having ultimate control over the Sith Imperative; a thousand year old plan that would see the Sith rule over the galaxy for 10,000 years. Palpatine hated him because he wanted all the power and glory for himself.
Palpatine was disappointed in Vader. That is, until he sensed Vader was trying to use his son Luke to aid him in taking him down at last. He envisioned three outcomes: One, that Luke would slay Vader and embrace Palpatine as his master. Two, that Vader would slay Luke and prove himself to be the worthy successor.
What we do know is he knew that the clone wars would happen, he says that in the clone wars. This means that he most probably knew about the clones and order 66. He doesn't know about anikin until about a week before order 66.
According to the novel 'Star Wars: Darth Plagueis' - now part of the Legends continuity - the witch detected the power of darkness in Sidious, so he offered Maul to him as a gift, because he believed that this way he would have a better life.
In the show Star Wars: The Clone Wars, Palpatine takes on Darth Maul and Savage Opress simultaneously. 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.
After finding out that the Queen had escaped the blockade, Darth Sidious was furious. He sent his Sith apprentice, Darth Maul, to find the Queen and the Jedi, destroy the latter, and bring the former back.
