Ruling over the remnants of humanity is inconsequential compared to ruling the laws of reality on Earth. Ever read "I Have No Mouth and I Must Scream"?Walter said:What Ganishka did wasn't a sacrifice. And what you propose doesn't sound very likely -- Griffith would rule over the... what, 5% left of the planet's population? That doesn't sound like a doable endgame for Berserk.
I think it would be very limiting, if not misleading, to subscribe to that way of thinking about these supernatural beings in a fantasy story. Were they once human? Certainly, but I think their lack of empathy for others doesn't weigh as heavily on their motivations as all the other factors at play.
Meaning... what? That he has to abide by that character's arc?
As for what I mean, I mean I think that the very existence and ruling of humans is probably inconsequential to him compared to his new goals in the long run. New "subjects" would be trivial to create if he transcends to a state of borderline omnipotence. If you are God, does it really matter that you banish countless millions of Angels and souls to Hell? Absolutely not. Also there is nothing wrong in drawing parallels between characters in fiction that bear similarities.