Muzan Kibutsuji was once a fierce and powerful demon. The demon king in fact! Those days are gone now as Muzan is barely even acknowledged as a living being and not a lab rat in the wretched lab that he's been imprisoned in. They degrade him, break him, make him crawl on his knees, act like he's just an animal. Few even know his name and he's glad that they don't as its humiliating to know how much he's fallen. He has no choice but to not lash out. He wont fight back even if you hurt him.