Exploring that shift in fanfic often feels less about creating a generic villain and more about stressing the core pillars of his character until they crack. Deku's entire identity is built on self-sacrifice, compassion, and an almost pathological need to save. An 'evil' turn usually isn't him gleefully setting things on fire—it's those traits curdling. The fanfics that stick with me are the ones where his analytical mind, that notebook-obsessed strategist side, gets weaponized not for rescue but for systematic dismantling. He becomes a colder, more terrifying version of AFO, because he understands hero society's weaknesses intimately.
I've seen versions where the bullying broke something irreparably, and he uses One For All not to smash buildings but to crush spirits, targeting the pride and public image of heroes like Endeavor. His 'evil' is a twisted form of his heroic drive: he's still 'saving' people from what he sees as a corrupt system, but his methods are merciless. It creates a fascinating tension—you can still see the ghost of the earnest kid in his meticulous plans, which makes the corruption hit harder. Those stories work because they understand his personality isn't being replaced; it's being inverted, like a photographic negative.
Endings in those fics are rarely happy. He either becomes a tragic monster who has to be put down by his former friends, a path that wrings out so much emotion from the class 1-A dynamics, or he wins and imposes a chilling, orderly 'peace' that's devoid of everything he once loved. The best explorations leave you wondering if this wasn't always a potential path, given the pressure cooker he was thrown into.