The following contains spoilers for My Hero Academia Chapter 350, "Bound To A Fiery Fate" by Horikoshi Kohei, Caleb Cook and John Hunt, available now in English from Viz Media
The Nomu from My Hero Academia aren’t the series’ main antagonists, but for most of the story, they formed a majority of the threat the League of Villains posed. Eraserhead's confrontation with Kurogiri revealed that these Nomu were actually reanimated corpses, a horrifying truth in and of itself, but the latest chapter revealed the process leading up to the creation of a Nomu might be even more sinister.
Along with the highly anticipated backstory of the League of Villains' Dabi, the latest MHA chapter gave some insight into how All For One conducted his affairs in the shadows. When Touya Todoroki was thought to have been lost to his flames, he was rescued by none other than All For One himself. Young Touya was then rehabilitated and finally awoke after three years to find himself in a strange facility surrounded by other children around his own age.
It's worth noting that none of Touya's peers in that facility seemed to be under any duress. They claimed that it was where they lived, and their mysterious masked caretaker informed Touya that they were to be his family as well from then on. One of the other children remarked that she'd arrived after Touya, implying that their little family continued to grow when All For One, or any of his associates, brought more children in. On the surface, it seemed like a paradise but a snide observation from an adult Dabi showed that it was anything but.
When Dr. Garaki presented Dabi with a High-End Nomu to test, Dabi remarked that turning him into a similar creature might be what All For One had in mind when the villain offered to restore his Cremation Quirk to full power. Garaki's inability to deny this was damning, and it inadvertently confirmed the terrifying reality of Nomus. A large number of the corpses required to make them probably came from these children.
Judging by the pattern of how All For One "adopted" both Shigaraki and Dabi, the other children must not have had comfortable past living situations. Essentially, All For One was preying on vulnerable children in society, enticing them with a myriad of rewards, including shelter or increased power, and then using them as the raw materials for his mindless soldiers. Since Dr. Garaki referred to Dabi and his other housemates as spares for Shigaraki, it's highly unlikely that either he or his master cared much for them past the viability of their Quirks.
The deception and murder of children is certainly not a line All For One is hesitant to cross, after all, Oboro Shirakumo was still in UA when All For One plotted the accident that took his life. Additionally, even Shigaraki who was given preferential treatment ended up being double-crossed when All For One stole his body and free will from him. It's no secret that All For One will stop at nothing to achieve his aims; The Nomu being reanimated corpses was enough to make him one of anime's most cruel villains but finding out that he would not spare even children sends him to an even higher dimension of evil.