Went back through the manga and found these panels mentioning Chisaki’s quirk. It’s interesting that, the quirk, Overhaul is mentioned as the ability to dismantle and restore....
Seems highly plausible that AFO and Garaki used his quirk to make Shigaraki's Decay. If so, Garaki probably performed experiments on Chisaki in the same nature as the ones Chisaki subjected Eri to…
It could explain Chisaki's lack of empathy towards Eri, and his extensive knowledge on quirk research, to the point of being able to create quirk-erasing bullets from Eri’s Rewind.

















