@Chizan It reflects out own worlds condition. Writers use demons and monsters and such to subvert the idea of "good and evil by default" that permeates our cultures. You see supposedly good figures all over the place and find them to be the evil ones too often to keep the facade going. The coat of paint of old stories and societies peels off with time and new stories are made that reflect that disillusionment.
It can represent anything and often represents everything. Racism and pretty much all ism's. Pretty people versus not pretty people. Popular people versus the downtrodden. It plays its part in all of the big stories from MHA, Black Clover, even one punch man to a degree. The idea is one of equality in a way. No one is good or bad, were all just people. Demons have been so strongly labelled as the bad guys that to use them as good guys is effective in showing people the judgment of entire groups as simply good and evil is too basic of a notion to actually be usefully true.
There is also a lot of push back since it used to be and still is super commonplace to show good people and bad people as black and white caricatures. The evil old hag and the pretty young princess. The dashing knight and the old fat noble or evil wizard who "looks clearly evil". Disney does this a lot. You get the idea. Media are still afraid to be anything but pretty but atleast pretty is getting more diverse and everything else is becoming more neutral.
Anyway if you want to rebel against the higher powers of the world, rebelling against the gods make sense. Who would be against gods? Well demons naturally. The iconic bad guys are now the only hope against justice gone wrong. Thats also where the edge-lords came from.