For all of his faults, Mayuri Kurotsuchi of Bleach is one of the most intelligent characters in the story, only surpassed by Urahara and Aizen (and barely). That said, one cannot skim over the various atrocities committed by this Shinigami, even if he claims to have done them in the name of scientific research.
Interestingly, Mayuri's personal arc is given quite a bit of attention in the manga-only Thousand Year Blood War Arc, starting with his rather unique chemical cocktails and concluding with his magnificent, albeit unnerving, modified bankai. It isn't a stretch to say that Mayuri helps save the day for everyone.
10 Mayuri Murders 28,000 Rukongai Souls Point-Blank
As the head of the SRDI, Mayuri is called to attend an extremely urgent matter, which he inevitably figures out to be the work of Quincy (who were obstructing the flow of souls by murdering Hollows willy-nilly).
Therefore, Mayuri directs his squad to randomly select and kill 28,000 people in the Rukongai districts, which would re-establish spiritual equilibrium. Although he does not ask for permission beforehand, Mayuri has a very good reason for what he does.
9 He Is The Only Captain Who Dares To Accuse Yamamoto Genryusai
Captain-Commander Yamamoto Genryuusai is astounded to learn of Mayuri's genocide and demands to know why the captain would commit such a risky act. Mayuri blames the entire situation on the old man, claiming that he had warned Yamamoto on several occasions, but his advice had been disregarded.
Mayuri goes on to claim that the current problems in Soul Society sat squarely on Yamamoto's head for not killing the arc's antagonist when he had the chance.
8 Mayuri Dons An Outfit As Bright As The Sun
Since the Wandenreich battalions were seen emerging from the shadows, not to mention Yhwach being pulled into some inverse dimension, Mayuri instantly theorizes that the villains require shadows to move around.
As a result, he builds a massive armor known as the Bakuhatsu Han'no Soko, which has two major merits to it. The armor is incredibly bright, thereby preventing shadows from forming around it; and it has been rigged to explode under enough pressure.
7 He Reveals The Product Of His Arrancar Experiments
Mayuri is faced off against the Zombie, Giselle Gewelle, who has converted her ex-colleague, Bambietta Basterbine, into one of her undead puppets. The two of them spar for a short while, which is when Giselle ups the ante by including more zombies in the fight.
Mayuri refuses to be left behind and releases his own "zombified" Arrancar, Luppi Antenor, Cirucci Sanderwicci, Charlotte Chuhlhourne, and Dordoni Alessandro Del Socaccio. To be honest, these Arrancar aren't exactly dead, which is why Mayuri can control them using electric shocks.
6 Mayuri Effortlessly Defeats A Zombified Hitsugaya
Giselle doesn't find any of the developments amusing, so she ends up bringing out the big guns: Hitsugaya as a zombie. The youngest Shinigami captain starts attacking Mayuri, and for the first few panels, it almost looks like Hitsugaya's winning.
Until he starts repeating the same few steps of the battle over and over again. Mayuri mockingly tells him that this time-loop is due to one of his iconic drugs before defeating Hitsugaya.
5 He Dares Aizen To Leave His Chair
Shunsui travels to the Muken to chaperone Aizen and his famous chair so that the latter could help them against Yhwach. Mayuri scornfully tells Aizen that there is no way his reiatsu, as incomprehensible as it is, would be able to destroy the chair.
Aizen takes this as a challenge and starts to exert spiritual pressure to prove Mayuri wrong—that is, until a Sternritter arrives on the scene and drives the narrative elsewhere.
4 Mayuri Faces Off Against Pernida Parnkgjas
Pernida Parnkgjas, the left hand of the Soul King, is no ordinary combatant. In fact, it can be argued that its raw power far exceeds Mayuri, who is Pernida's final opponent.
For most of their battle, the Sternritter has the upper hand, even though Mayuri reveals some pretty fancy techniques in the form of Hirenkyaku and Jabarakaina. Pernida's The Compulsory is strong enough to incapacitate Zaraki Kenpachi, but it fails to instantly defeat Mayuri.
3 His Terrifyingly Modified Bankai: Konjiki Ashisogi Jizo: Matai Fukuin Shotai
Mayuri is pushed to the point of bankai, but this time his zanpakuto takes a rather unusual form: an enormous baby-like figure. He tells Pernida that Matai Fukuin Shotai's nerves are separated into over seventy thousand layers, partially neutralizing The Compulsory.
Unfortunately for Mayuri, not even his modified bankai is capable of taking Pernida down, which means the task is ultimately left to his Vice-Captain, Nemu.
2 Mayuri Explains The Nemuri Project
Mayuri reveals that Nemu Kurotsuchi is a kind of artificial soul composed of "konpaku cells," but a flashback shows that he always had paternal-ish feelings toward her.
Nemu is actually the 7th of her kind, although what happens to the previous 6 is unknown. After Nemu effectively defeats Pernida and saves Mayuri, the latter leaves the scene with her brain matter, suggesting that he has further plans with it. Those plans turn into the eighth of the Nemuris, Hachigo.
1 He Attempts To Destroy Yhwach's Vestigial Reiatsu
Ichigo Kurosaki kills Yhwach with Uryu and Aizen's assistance, but the Quincy Father promises that his reiatsu will return in a decade to seek vengeance.
Seireitei is well aware of the situation, and the first captains to reach the supposed location of Yhwach's return are Mayuri, Byakuya, and Suì-Fēng. As the three of them notice a swirling black substance, they prepare to defend themselves, but it suddenly disappears without any explanation.