I'm just thinking with all their actions to suppress said-emotions and even apathy itself, is still enough to 'feed' the demon. Not to mention, nothing is truly infallible.
Also that demon king part has definitely had enough time to integrate itself into the Spirit Tree.
Thanks for all the translation and hard work! Really appreciate it! Please keep up the hard work!
EDIT: I done goofed. I just realized I wrote the wrong comments for a different manga

I should've not binge read manga all at once. I don't know how to delete this comment so until then, I'm sorry for throwing people off and doing the unthinkable of posting in the wrong forum thread. I was reading "Ninkyou Tensei: Isekai no Yakuza Hime" before this and had too many tabs open it seems. MY APOLOGIZES