Kotaro Yanigasawa, alias Shido, was a Mad Scientist focused on utilizing anti-matter in a human body, and the reason Koro-sensei may destroy the world. To this end, he used the famed assassin known as the Reaper as a human guinea pig in painful experiments, using his own fiancée as their liaison since Yanagisawa figured she was worthless as a hostage. When the Reaper escaped, destroyed the lab and took Yanigasawa's eye, the scientist formed the identity of Shiro and plotted his vengeance. He implanted a boy named Itona with the same tentacles and later abandoned him to die when he'd served his purpose. Mutating the assassin's former protege into a monstrosity, he tested his new success by having him annihilate a giant building—and anybody inside—before attacking Koro-sensei and his students. Having the second Reaper attack Koro-sensei, Yanagisawa opted to then have the students attacked, gloating how he'd kill every single one of them painfully and Korosensei would be forced to watch his class die before dying in despair. When his former fiancée's sister attempted to help Korosensei, Yanagisawa had her impaled through the chest and gloated how he'd intended to use her as a "replacement" for her sister.
Akira Takaoka is a brutal military trainer known for torturing his students with sadistic relish while he breaks them mentally. During his brief stint as Class E's trainer, he is willing to brutalize any unruly student, regardless of gender. After being ousted from his teaching job, Takaoka devotes himself to revenge. After stealing money to buy a bio-weapon and hire hitmen, he orders for Class E to be infected with a virus to cause a horrifyingly painful death as they slowly suffocate under the weight of tumors. The few students who escape this fate are targeted for even worse fates as Takaoka intends on burying Kayano alive in cement in a bathtub full of material lethal to Koro-sensei, which would force him to sacrifice his life to save her. In his rematch with Nagisa Shiota, Takaoka makes the boy grovel before him and apologize for his previous humiliation of him in exchange for the antidote to the virus only to then blow up the antidote right in front of him, laugh at his anguish, and challenge him to a fight to the death in which he savagely beats him down, stating his intent to rip him apart and keep his severed remains as a trophy. To drive in his victory, he gloats that he even might use the reward money to purchase middle-school aged kids to murder on a regular basis as some twisted celebration.
Magnificent Bastard: Gakuhou Asano was once a caring teacher who sought vengeance on the bullies who drove one of his students to suicide by getting them addicted to gambling, ruining their lives in the process. Becoming the chairman of Kunugigaoka Junior High School, Gakuhou established a system that would make sure that 95 percent of the students would be successful, by demeaning the lowest 5 percent, Class-E. He would make sure that the top 95 percent would constantly triumph Class-E over academic and sports, by subjugating them to his brutal teaching methods. Willing to prove that his system is flawless, Gakuhou challenges Koro-Sensei to open one of the five handout, before the grenades inside detonate, with four of them being lethal to Koro-Sensei. When Koro-Sensei fills out four of the five handouts, Gakuhou opens the one lethal to him in an attempt to take Koro-Sensei with him. Having a change of heart after Koro-Sensei saves him, Gakuhou accepts his loss with grace before resigning as chairman.