Warning: Spoilers Ahead!
I just finished Berserk, and I'm left sitting here in confusion. I tried finding the answers on the wiki and forums, but had no luck. Came here in hopes of finding a Berserk fan online.
Griffith dream was to become a ruler of a kingdom, but at what point did his dream all go to hell? What exactly am I missing in the story that says "My dream is to rule humanity!"?
How did demons originally get involved in all this...?
If you recall in (the original run of) the anime, they call the Behelit the "Egg of the King". There are other Behelits too. In the manga, something similar happens to many other people. Typically, a demon in disguise intervenes when someone is down on their luck and gives them the Behelit. The Behelit warps their fate, giving them everything they wanted. Eventually, fate twists violently and brings them back to despair. When their blood comes in contact with the Behelit (which tends to happen at this low point), the God Hand appears and offers to grant a wish in exchange for a sacrifice. This process turns them into monsters known as Apostles. It's heavily implied that something like this previously happened to the Snake Baron in the first episode of the anime, as well as all the monsters in the last episode. It's been a while since I've read it, so the details may be off a little.
Basically the Behelits are human bait, leading people into situations where they will lust for power enough to embrace being a demon. Griffith's dreams and fate were also manipulated, or at the very least the darkness in his heart was cultivated until he chose the outcome.
The entire anime is just a small part of the entire storyline. It was canceled for obvious reasons.
Warning: I just read the manga so I don't know if it happened the same way in the anime.
First of all, there were three facts that were given when Guts was in the Band of the Hawk:
1) A Behelit was given to Griffith, that saved his life several times.
2) For him, his dream was more important than anything.
3) He was always considered as a special person, a chosen one that could do anything.
So, we can say that he was chosen by the God Hand. But the moment that precipitated things was:
when Guts decided to leave the Band of the Hawk, at the end of the war. Guts was an important piece in his plans to conquer the throne. So, Griffith decided to fight Guts to force him to stay in the Band of the Hawk but lost and Guts left him.
After that, Griffith looked desperate so he rushed his plan and
he was arrested when he took the virginity of the princess. He stayed in the darkness of an underground jail for a long time where he met the God Hand (Dream? Hallucination?). When Guts and the others freed him, he was already influenced by the God Hand.
Because he had a broken body, he knew he couldn't rule over a country with human methods. So he thought that his dream was more important than the lives of his comrades and decided to sacrifice them to become the fifth member of the God Hand.
Griffith's dream was to become a ruler of a kingdom.
While it has always been true for any form of Griffith (purebred human, angel Femto, reincarnation into human) that his goal is to have a kingdom, he has never mentioned the boundaries of or the terms for his kingdom. That is, a reader in early chapters of the manga could think of it as the kingdom of Midland; in middle chapters, as Empire of Holy See and of Ganishka; in recent chapters, a kingdom encompassing both Human and Astral World.
But at what point did his dream all go to hell?
As it is already clear in the manga, this dream of having his kingdom is the reason why Griffith in his human form lived, later took rebirth into an Angel (to get rid of human body restraints) to start his expedition of having a kingdom again (because of being feeling lost in the jail for so long when he was purebred human), and had reincarnation into a human to claim human world from the existing emperors. Accounting the development in recent chapters of the manga, there is no doubt that this dream has not been abandoned but has been invigorated with new plans.
In your words,
the dream did not got to hell but the hell has been brought to earth by him to rule both simultaneously from Windham.
How did demons originally get involved in all this...?
Some part of the following text is complete opinion of mine.
Humans follow the fate set forth by The Idea of Evil and the Behelit which the chosen ones receive draw them to the currents of causality, thus, reaching a point of despair and malice, that they decide to offer sacrifice to get rid of this feeling.
Turning into an Apostle doesn't end fate but marks a watershed moment. They are still headed for the destiny set forth by fate knowingly (like Emperor Ganishka thought) or unknowingly (like the tragedy with Count).
As for Griffith, his fate was set by The Idea of Evil and the Crimson Behelit made sure that he remains drawn to the currents of causality as answered by Gunfulker.
Now, to the question about involvement of the Demons.
The Apostles might knew or could have received the information that their will be a feast on eclipse, or an angel, a member of God Hand, will be reborn, or it could be both. So, they came to the spot en masse.
The aforesaid conclusion can be deduced from the following events in the manga:
It was not shown in the manga but can be concluded that, since Zodd was seeking the strongest one to fight (as he witnessed the dream of a white hawk flying in a dark sky) he was defeated by Femto, thereby, Zodd became the first member of Griffith's army. It cannot be said how Locus, Grunbeld and Rakshas came to know of Griffith's presence on Earth, but they chose to join him voluntarily as witnessed in the manga.
It has also been witnessed that a demon cannot stand against Femto, irrespective of their wish. When Ganishka (extremely powerful Emperor and a demon) encountered Griffith the first time on the battlefield he felt been carried away by latter's presence, comforting voice, and the blood of demon realm that flows through Griffith too. Therefore, it can be concluded that while an Angel doesn't control a demon willingly, a demon cannot directly fight against, as well as oppose an Angel.
Hence, it can be ascertained now how did demons get involve with Griffith into his War-Demon army.
Note: The question has been answered by accounting episodes 01-337.
Griffith lost his way after Guts won the fight and only could think about how he lost to him and that Guts wasn't an equal. So when he was with the princess, he made a mistake and got thrown in the prison where he sat in darkness, broken and beaten.
At that point where he felt completely lost and broken, he felt he had no other way of making it out alive. However, Guts showed up and saved him. In the moment when Guts saved Griffith, he felt as if he was being punished and made his choice to sacrifice them in order to become a ruler over the land.
Griffith did that in order to become a god king and beat Guts once and for all.
