My Hero Academia likes to portray its villains as sympathetic, or at least with a chance to be redeemed, but there's at least one major villain Kyudai Garaki, who has proven he's deserving of no sympathy at all, and it's a manga bonus page that puts the final nail in his evil coffin.
Dr. Kyudai Garaki is All for One's most loyal follower; he's a mad scientist and the one who made it possible for the Symbol of Evil to live for so long in the first place. He's also responsible for creating the Nomu, the undead minions of All for One who are packed to the gills with powerful Quirks and raw strength. It's explained pretty clearly in the story that Nomu are made from dead bodies, reanimated by Garaki using a combination of Quirk powers and science. That's bad enough, but there's one particular Nomu that has a much, much closer relationship to Garaki than any other, and that's because it used to be his grandson.
Garaki's Grandson Became the Winged Nomu
In the Hero Killer Stain arc, a trio of Nomu are released on Hosu City and immediately began wreaking havoc. The Winged Nomu aimed at Izuku Midoriya in particular, picking him out of the crowd and trying to drag him away. The Nomu is ultimately killed by Stain, and little more is said about it in the story. In the bonus materials of My Hero Academia volume 7, however, is a note from series creator Kohei Horikoshi, accompanied by a sketch of one of Bakugo's friends, a boy with dragon-like wings who used to bully Midoriya as well. Accompanying the image is a statement asking why this Nomu chose Midoriya, of all people present, and a promise to tell the story of the Nomu in the future.
The image strongly implies that this former bully of Midoriya's, who was named Tsubasa and appears on the first page of the manga, became the Winged Nomu seen in Hosu. When Garaki is first seen, he's in disguise as a "Dr. Tsubasa", diagnosing Midoriya as Quirkless. Another databook states that "Dr. Tsubasa" is the grandfather of the Tsubasa kid who used to bully Deku. This means that not only did Garaki use a child in his experiments to create Nomu, but this child was seemingly related to him. While villains like Toga Dab,i and Twice might have tragic backstories that put them on the path of evil, Garaki is known to have once been a scientist who was laughed out of his field for creating the Quirk Singularity Theory, and then blindly followed All For One, creating mass-murder weapons for him.
Dr. Garaki Is A Villain Beyond Any Possible Redemption
Turning a child - possibly his own nephew - into a monster really makes Garaki stand out from other My Hero Academia villains, except for his master All For One. Dr. Garaki often treated the Nomu like they were his children, and this single page offers a chilling explanation as to why that's not otherwise found in the main manga. Garaki already came across as the least sympathetic follower of All for One, given the horrible things he was responsible for, but this reveal really escalates him to a new level of evil. All for One may always be My Hero Academia's most vile villain of all, but Dr. Garaki is an easy choice for number two.