Darth Maul’s frightening visage was ubiquitously encountered before the release of The Phantom Menace. From the moment he was first introduced in the trailer, it was clear that he would be a success, and he has subsequently become one of the most-used characters in a vast array of merchandise.
So when he showed up in only a few episodes, with just 31 words of dialogue, and was cut in half during the final confrontation, it was a letdown. Maul’s death at the hands of Obi-Wan Kenobi is still regarded as one of the most significant moments in Star Wars history.
Darth Sidious spent so much time teaching Maul because he was a throwaway weapon, as fans of r/StarWars have observed. The OP adds that Darth Sidious trained Maul since he was a kid, but he’s less powerful than Dooku and Vader and lacks political or military talents.
Maul is described as being highly competent, yet we don’t see it in the films. After all, in The Clone War and Rebels, we learn he has seized control of Mandalore and runs a vast criminal organization, not to mention regaining his sanity after being sliced down the middle and thrown into a huge pit.
Some people argue that Sidious was, in fact, devastated when Maul died. In Marvel’s Darth Vader comic book, the Emperor considers Maul to be a major loss and a severe setback for his plans.
After all, Maul is a highly capable agent and assassin for the Emperor. We see Sidious resort to hiring various bounty hunters and assassins in order to carry out his plans, with mixed results. If Maul had been around, those strategies would have been completed far more effectively.
Darth Maul hasn’t been seen in live-action since Solo: A Star Wars Story, though the poor box office appears to have halted those plans. He’s a fascinating Star Wars antagonist who deserves more than 20 years on screen, so here’s hoping he reappears in anything.