WARNING: The following contains spoilers for My Hero Academia #282, by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
The grim war between the heroes and Paranormal Liberation Front of My Hero Academia proved itself to be the darkest arc in the manga yet, with the situation growing direr for the heroes in each chapter with no let-up. The battle against an awakened Tomura Shigaraki, in particular, was terrifying to witness, as thanks to his new, nomu-derived synthetic body, he was able to fend off the heroes even when his Quirks were nullified. It took the combined power of Endeavor, Deku, Bakugo, Gran Torino, Ryukyu, Aizawa, Manual, and Rock Lock to stand against him, and even then the heroes ultimately lost.
In My Hero Academia #282, we learn the fate of the Quirk-erasing bullets Shigaraki fired off in the previous chapter's cliffhanger, as well as the extreme lengths Aizawa is willing to go to stand between Shigaraki and the public he's sworn to protect. To prevent a Quirk-erasing bullet from taking root in his body and stealing his Quirk, Aizawa unflinchingly saws off his own leg. Future events in the manga prove this decision -- in true character for Aizawa -- was indeed a rational one.
The Context For Aizawa's Decision
Chapter #282 of My Hero Academia begins with Deku unleashing a 100% Wyoming Smash on Shigaraki -- hoping to bring the battle to a decisive end -- and breaking the villain's jaw. Ryukyu is providing support by pinning Shigaraki to the ground when she notices that he's aiming two Quirk-erasing bullets at her fellow heroes. As someone who participated in the Overhaul raid during Season 4, she's able to recognize the Quirk-erasing bullets for what they are in a split second and shout a warning to everyone else.
The other heroes react, but not quickly enough to save Aizawa from the incoming bullet aimed his way. It pierces him through his wounded leg -- a leg which had already been crushed by falling debris during Shigaraki's earlier Decay Wave and led to Crust sacrificing his life in order to save Aizawa's own. Crust's sacrifice, alongside Aizawa's vow to live to take custody of Eri and see Class 1-A's graduation, all likely steeled his resolve for the decision he makes next.
Why Aizawa Saws Off His Own Leg
Brandishing his knife, Aizawa stabs down on his wounded leg and severs it from his own body. Thanks to Ryukyu's warning, he thinks, "I can handle this ... nice and rationally ... without any hesitation." He pictures Eri while amputating himself, the abused child he was able to take into his custody solely because his Erasure Quirk meant he was one of the few people able to keep her dangerous, uncontrolled powers in check. Without his Quirk, it's likely Eri would be taken away.
It's a desperate move, but one that pays off. Aizawa is able to prevent the Quirk-erasing bullet from interfering with Erasure, so the heroes' best weapon against Shigaraki isn't lost. Should Shigaraki regain full use of All For One and Decay without any threat of interference from Erasure, the remaining hero forces are at risk of being decayed en masse. The brutal wound Aizawa inflicted upon himself to prevent this outcome proves that he's not only the heroes' MVP in My Hero Academia's War arc, but that he's doing his utmost to honor the sacrifices of everyone who died to get him there.
Aizawa's sacrifice buys the heroes time, but it's up to Deku and company to make it count.
The War's Aftermath & Aizawa's Fate in My Hero Academia
Though Aizawa had to make a hasty choice to sacrifice his leg, later events in the My Hero Academia manga prove that even in the most desperate of circumstances, he's able to remain cool-headed and think things through rationally. Not only did his sacrifice save the lives of his fellow heroes, but it also may not take Aizawa away from fieldwork for very long at all. Later in the manga, we see that both he and a similarly injured Mirko have been hospitalized and fitted with prosthetic legs. Mirko is said to be adapting well to her prosthetics, and even Aizawa is making do.
Furthermore, the possibility of a complete recovery cannot be ruled out. Eri was shown at the end of the War arc to have sufficiently mastered her Quirk to the point of restoring Mirio's. This means that once her horn regains its charge, she'll likely heal her adoptive father's leg as well.
If Aizawa ultimately walks away from this with a restored leg and the knowledge that he protected everyone he could from Shigaraki, it'll prove that he understands how to play the odds better than anyone else in My Hero Academia.