So the demon king angry and vengeful because 'her' people got killed
Tbh... 'her' story was a lot deeper that it appeared 'cause what she wanted was life. So every time she devoured a life she could recreate it to be, according to her understanding, 'whole'. I believe that she actually was a type of saintess in her own way because she returned the memories to the being she recreated. Then when the people she consumed or took in, were attacked and slaughtered by their own fellow humans in a manner that even beasts do not behave, 'she' was horrified and cursed them to be consumed by their own malice. Then in turn 'she' was also consumed by grief, resentment and anger turning 'her' into a demon lord.
That is one way of looking at it.










She's gonna be very pretty when she grown up (≧∀≦)