If i recall this is like volume the end of volume 4 of the LN and the start of volume 5 (total is 16 volumes), though the manga so far has skipped half the story leading up to this part in the manga. spoiler
To me it seems like the world has a very very pure black and white idea of what is good and bad, the demon lord is undead = evil.
Its like the word itself is corrupted and very set in its way, and does not accept anything that's against the way it wants the world to be.
TL;DR the world is a tyrant
They rebelled due to the other split personality minds that she had, and were just following what those "minds" were telling them to. When she took them out that made her the only one that can control them.