WARNING: The following contains spoilers from the My Hero Academia manga by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
Overhaul, aka Kai Chisaki, is about as irredeemable as characters get in My Hero Academia due to his horrific abuse of young Eri. During confrontations with both Lemillion and Deku, Overhaul spews vile, grandiose rhetoric about eliminating Quirks from the world. Such behavior is typical of villains when battling someone who opposes their ideals, but it has the unfortunate side effect of causing the audience to lose sight of their motivations.
Kai was a child living on the streets when a man known only as "The Boss," leader of the Shie Hassaikai, took him in and raised him. The only person Kai has ever truly cared for, he has always felt an incredible debt to the man who saved him. By joining the Shie Hassaikai, Overhaul sought to repay his debt by restoring the criminal organization to its former glory, no matter it what took. Even though The Boss was against his violent and immoral methods, Kai refused to stop.
When The Boss refuses to use his granddaughter Eri's Rewind Quirk to create a Quirk-destroying weapon, Kai puts him in a coma in order to assume control of the Shie Hassaikai. In Overhaul's mind, he is repaying his debt and can't comprehend how The Boss doesn't see it that way. Without knowing it, Kai is acting out of love, unaware there could be another way to show his gratitude. The only way he can think of to say thank you is by restoring the criminal organization, even if it means taking control away from The Boss.
Kai is also an extreme germophobe, breaking out in hives and growing angry whenever he gets slightly dirty. He refuses to even breathe the same air as the people around him, hence his trademark mask. Kai also considers Quirks an infection of the entire human race that gives people delusions of grandeur by becoming heroes or villains. However, he fails to see the contradiction between his beliefs and his actions.
Overhaul's desire to rid the world of Quirks stems from the approach he initially wanted to take with the Shie Hassaikai. To return it to its former glory, Kai wanted to use Eri to destroy the Quirks of rival gangs, forcing them to submit. He aims to repay the kindness he was shown by The Boss, and he's willing to destroy the world if that’s what it takes.
By the time he resurfaces in Chapter #312 of the My Hero Academia manga, Kai Chisaki is a broken man. Having lost both of his arms, he is unable to use his Quirk. When All For One stages a prison break at Tartarus, Overhaul is among the escapees and joins Lady Nagant on her mission to capture Deku in exchange for being taken to see The Boss. Surprisingly, he doesn't care about the boy who defeated him -- all he wants to do is tell The Boss that he's sorry.
Kai exclaims The Boss is the only person he has left and that he must apologize. He doesn't appear to be sorry for his heinous actions or any of the lives he took, only that he failed. The debt he feels he owes is the most important thing in the world to him, and all he's ever wanted is to prove he is worthy of The Boss's kindness. Even when Deku offers to take him to The Boss as long as he apologizes to Eri first, Overhaul's only concern for Eri is that she is still the key to his desires, maybe even restoring his arms.
Acting out of a twisted, delusional form of love, Kai Chisaki feels as though he has lost everything as a result. His failure to repay his debt may make him feel like he was unworthy of being saved in the first place. The tragedy of his character is that the monster he became in My Hero Academia was not what The Boss wanted for him. He only wanted Kai to be an honorable man.
