It's probably muzan's bad luck that he got there. It's interesting that he didn't even feed though, he simply tried to turn them into demons and most of them were lucky enough to not survive the process.
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They don't know he'll die on his own, just that he isn't splitting apart anymore. The only way for them to become sure is if they either kill him or stall enough for the sun to do it. Tamayo said he's going to die now, but she didn't say the poison alone was sure to kill him. If she does think that, she could turn out to be wrong. Since this is a shonen, I think that Tanjiro will definitely need to finish him off.So, Muzan can be left alone and he'll die??..I really didnt like anything about this chapter. Not the textless pages of Nezuko turning human & remembering a few things or ppl she's met, thay took up way too many pages. & the whole Tamayo drugs taking effect now, the way they are is...