Uh, no.
You very much misunderstood that.
Goetia's entire plan hinged on a simple thing - he pitied humanity and saw their struggles as pointless. Thus, he decided to end them entirely. Killing everyone across all time would be fine, because all he'd have to do was use the power generated by burning thousands of years to shoot himself into the past, becoming the new God of Humanity from the onset, thus creating a world where humans would never struggle. (Essentially similar to LB3 or LB5, actually.) In doing this he conscripted all of the Demons and did a shitload of setup, brushing off their differences in getting them to work together, and prepared for thousands of years to enact his scheme.
It's just, he didn't actually understand humanity. And the only pillar who did was Flauros, who only had a very brief amount of time in charge, and who Goetia basically went "na fam leave them be" to after Septum because he didn't see how they were a potential threat. Now, mind you, he's unfathomably strong and has precisely 1 weakness that he thought he covered, so it shouldn't have been an issue, but, uh... well.
That is, by the way, why Goetia's last form changed from "King of Demons" to "King of Men". He finally, finally understood why humans struggled, and began acting in the manner of a human himself... by futilely burning the last of his power in a final "FUCK YOU" meant to kill Chaldea, in spite of the fact that he was already dead. Truly a human in the last.
I'm not actually seeing how that contradicts what I said? Especially combined with his belief that Mash should understand his views about the 'eradication of death' due to herself being designed with a short life span that only exists to serve others.
Also I'm pretty sure it was implied, if not outright stated that the reason Goetia couldn't understand humans was because he was immortal, and humans
aren't. He became the Human King after Solomon's sacrifice and Ritsuka's defeat of the Demon Pillars left him with a limited, mortal existence, and his final words are about how the briefness of human life is actually a lot more interesting\meaningful than he had thought. All of which suggests that the key thing that Goetia was missing the whole time was
mortality.
It also wasn't a final 'fuck you' or meant to kill Chaldea; the Human King never expected to win that fight. He fought to show respect to his enemy, the magnificent opponent that had taken everything he threw at them and defeated him utterly, and he fought to experience the joy of
living, something that he was only capable of due to his new mortal existence. IIRC it's explicitly stated that he held no anger or resentment towards Ritsuka for defeating him, because even if he
did win, he was still defeated and the Pillars were still gone, so he fought not to kill Ritsuka, but to discover what it felt like to be
alive.
None of which changes his original plan to burn all of human history to provide the energy needed to turn himself into an unlimited, ultimate being, a 'God' in every practical sense, something that required first becoming the sole existence on the planet, presumably due to conceptual magic reasons.
e: Yeah, Fate Grand Order Material IV:
Human King Goetia said:
The "last opponent" who, after the last battle happened and everything had been settled, appeared in front of the protagonist from inside the Temple.
However, by gaining lifespan he became able to comprehend humans, something that Goetia until now was unable to do; he had achieved the state of "human king", a wise king to surpass even King Solomon.
He did not operate only with the violent emotion such as rage and resentment belonging to the previous Goetia, but also with the natural emotion as a human that he now had possessed, as he had arrived to understanding.
He held no anger towards the protagonists that had defeated him.
Even if he killed the protagonists, the fact that Goetia had crumbled and had been defeated would not change.
That last standing of his is simply "the last obstinacy as the one who had persevered until this moment".
Both in spirit and in action, rather than the last boss he has the feel of the main character instead.
His body crumbled bit by bit during the battle; it was regrettable, but he did not lament over it. His only real feeling was "So this is what being alive means. What life means."
"To show respect to this magnificent opponent, all the while swallowing the emptiness of losing everything, and clash as enemies until the end."
That was what granted unto him, the sole greatest reward for his 3000 years of grand undertaking.
Mortality is what let Goetia understand humanity, and the final fight was about feeling what it meant to be alive and showing respect to the one that had defeated him.
