By no means would I call myself an expert in the lore of DAL, but what I understand is that Mio gave Sephira crystals to random people to "polish them" (aka make them compatible with a human host). In this case, she gave an "unpolished" crystal to Sawa, which turned her into a monster that Kurumi killed. Then the crystal was retrieved and Kurumi was able to see that the "monster" she killed was actually her best friend. That's why she almost inverted.
@@bloopeeng Yeah you nailed it. In the LN, Mio openly admits to sacrificing a whole bunch of people to refine the crystals to adapt to human bodies since she could only make artificial spirits and not pure spirits like herself and SPOILER
But from what I see, Mio has more character developments than Muzan. The Demon King just has the desire for himself to be immortal (for no interesting reasons), while the First Spirit only wants to make her lover to live forever like she is. And she also changed her mind from killing all the Spirits to helping the main character to save the world from DEM.
