We know the Demon Lord didn't always exist, and that the old Sun God supposedly opposed it and was devoured by the first Demon Lord/first Apophis...
We alson know, thanks to Monster Girl Encyclopedia World Guide - Side III, that there is a big change of tone in the Chief God's scriptures... it's all about being nice and warm and suddenly starts going about killing monsters and chastity.
We could attribute the change of tone to the current Chief God being a newbie, but since the monsters appeared long ago, starting the cycle of mutual genocides, the part about slaying monsters had to be introduced long before she took the throne...
Could it be that the current Chief God is in fact the third one to wear the crown?
Like:
1st Chief God creates the world, and it's mostly peaceful.
2nd Chief God decides the world is overpopulated and creates the Demon Lord and the cycle of genocides.
3rd Chief God gets her ass kicked by the current Demon Lord...
That reminds me of Greek Myth.
In Greek Myth, humans were molded from clay by the titan Prometheus during the reign of the titan Chronos (Prometheus was Chronos' nephew). Chonos was a monster to his children, but he was nice to humans, and the latter lived a Golden Age: They didn't grow old or sick, they didn't have to work, there weren't wars, and they mingled freely with gods and titans ,who didn't see them as a lesser order of beings, but as weak cousins.
Astrea, the goddess of Justice ruled side by side with Chronos, so there wasn't crime or oppression either...
But Zeus rebelled and dethroned his father Chronos. Zeus divided the universe by spheres of influence: The sea for his brother Poseidon, the Underworld for his brother Hades, Heavens for himself, and Earth was common ground, but he also commanded that humans and gods should live separately from now on; also, humans produce food and offer sacrifices to gods.
Prometheus didn't like that, so he tricked the gods into accepting the skin and bones of sacrificed animals as their share, while mortals would keep the meat.
Zeus was enraged, and took fire from humans, but Prometheus stole it back with Athena's help.
Zeus was pissed: He didn't punish his beloved daughter Athena, but he chained Prometheus to a mountain and had a vulture eat his liver every day for all eternity. He also sent Pandora to spread diseases and disasters among humans.
Humanity degenerated: The Golden Age ended, and was followed by the Silver Age and the Bronze Age. At the end of the Bronze Age Astrea left Earth (hence, Justice ceased to exist in the mortal world), the Bronze Men waged war until near extinction, and Zeus unleashed the greek version of the Great Deluge to wipe the survivors.
But Prometheus's son Deucalion saved Humanity building the Great Ark, starting the Iron Age, the current one.
Now, what if the first Chief God, the one before Demon Lords was Chronos? Life was peaceful and good then... All humans were similar to current heroes (no limiter to their development), and monsters weren't a threat to them.
The second Chief God was Zeus. Zeus got pissed at humans and unleashed all kind of disasters: He created the demon lords and ordered Poseidon to provoke storms and typhoons. He put a lid in the development of humans, so only his heroes could become strong, and had the first Demon Lord/Apophis devour the Sun God, who had opposed his plans.
The third Chief God, the current one, is Athena, greek goddes of Wisdom, Civilization and War. She is a virgin goddess who hates sex, but loves heroes and promotes civilization.
In Greek Mythology, Athena was supossed to be a male god and Zeus's sucessor, but Zeus genderbent her into a girl to prevent her from ever becoming a rival (the Greek were very sexist... a woman ruler? hah!). But in the MGE world, where Poseidon and Ares are female, Athena being a girl wouldn't make much of a change, so she could have inherited her father's throne anyways...
Athena always opposed her father's plans to punish humans (she got away without punishment because daddy's girl...), so the MGE's Athena may have chosen to get rid of monsters altogether, which may explain why the monsters hurried to choose a leader... somebody leaked Athena's plans, and they stopped fighting among themselves in order to stop the gods...
That would also explain why DE is so corruptive after Lilith's changed it... she did it as a counter to Athena's plans. She could have made monsters into monstergirls without making the DE able to corrupt human women into monsters, but she needed a weapon to make monsters basically unstoppable.