I believe that Geryon was a nice human much like Houtarou from an era like the Ancient Egypt, Italian Renaissance, or Ancient Greece.
But then, after being abused by a greedy and domineering noble and having his family killed, he grew to despise humanity as a mysterious form of darkness made him want to kill the noble he hated so much and encouraged him to make said scumbag suffer in many ways more than imaginable.
Then using his newfound dark powers, he possessed the noble and erased his consciousness, making the scum’s body his.
Then in excitement, he gave in to his newfound unhinged and broken psychopathy as he began to hate humanity in extreme levels, seeing them as nothing but beasts of malice alone.
With his newfound misanthropy, he modified his body to make it an immortal, omnipotent vessel that would toy with the lives of others.
And he even did so with the Chemies, because he gave up love a long time ago.
And because he saw humans as nothing more than ugly, foolish beasts of nothing but malice, he believed that humans of Planet Earth of every generation live by the mantra that Flowey from Undertale believed in.
“In this world, it’s kill or be killed.”
Thus creating an obsession of turning the world into gold.
And he may be a literal pervert, because he had captured many young children to become monster homunculi, with their memories of their past lives erased for good. Both males and females.
I know this is a stretch, but the picture book named “The Great Path of Alchemy” revealed a demon who captured children. This demon could be what the old generation of an ancient civilization imagined what Geryon was, a demon from the darkness.
And these three children, could be the Three Dark Sisters themselves. They must have been brainwashed and permanently transformed into hommuncili, reminiscent of Father’s Hommunculi from Fullmetal Alchemist. Young ones whose memories of their previous lies erased, replaced with different aspects of Geryon’s malicious personality:
Lachesis - His lustful desire to see humans suffer in despair for all eternity.
Clotho - His wrath against the humans for killing and exploiting others.
Atropos - His cold hatred against all humanity; seeing them as nothing but mindless beasts who serve no worth and no purpose but to be exploited.
With these three parts combined; he made them forget about things like the ability to understand why the life of all living beings, humans and animals matters so much.
Because when we see them; they see human life as nothing but expendable pawns and ugly beasts that deserve to die and be destroyed, because they were literally perverted with Geryon’s mantra of “Kill or Be Killed”.
I say all of this because even after Geryon was put back into the Door of Darkness, I don’t think it’s over for him.
He will come back, and he will have a vengeance.
And maybe more of his origin story of why he hates humanity will be revealed.
Because Atropos just retrieved his Rubik’s Cube, which is still glowing.
Just what is beyond the Door of Darkness? What kind of world is it? What is that three-eyed monster that Geryon called, “El Dorado” back there?
What drove Geryon to have a huge misanthropic hate for us humans? He reminds me of Flowey the more I look at him and think about him.