As Griffith fell into the abyss, he realized he wasn't alone. He was drifting towards a giant lump of flesh cloaked in whirlpools of energy and staring at him with a million eyes. This was the Berserk universe's God, or at least a part of it.

It was a swirling mass of every negative emotion that cried out for meaning. Those feelings ran deep, deeper than joy and happiness, deep enough to create this great being. It was the collective unconscious of the species, and as such it was a very human place. By this logic, the sacrificial ceremony wasn't about severing ties with humanity at all. Rather, severing the superficial individual ties and developing a stronger connection to the mass consciousness in order to see the bigger picture.

The Idea of Evil was created by humans, for humans. It existed to explain all the misery, suffering, and absurdities in the world.  It was willed into existence by humanity, and shaped destiny to reflect the desires of humanity as a whole. These were not necessarily conscious wishes. The power of the rational mind is weak compared to the irrational force of feelings. Feelings of vengeance, hatred, and despair that most people try to hide. But they don't go away, they fuel the Idea of Evil, and create a world that reflects them.

When Griffith understood this, he was at first shocked that something this terrible would be humanity's will for him.

Yep, this was in the works for a veeery long time.

"We are all at the mercy of a great tide... fate, or whatever you wish to call it... and we all disappear in the end, our lives spent, never even knowing who we were." Griffith, Vol. 3 Ch. 3

Griffith was always fascinated by the whole "destiny" idea. He knew he was special, but he was afraid he would disappear before he ever truly understood who he was, just like everybody else. He had a dream, and he wanted to make that his destiny, but he still longed for confirmation. But after that dream crashed and burned to the ground, he thought he was lost. Was he following the wrong path all along? Would he die ignorant like all the rest he crushed along the way? He thought he would. For a little while, he even wanted to.

"...The destiny... My... God! What do you want from me?!" Griffith, Ch. 83

Yet here was God itself! He could ask these questions straight to its face! It gave him hope and motivation.

Mmmm... "Chosen One." Griffith likes being chosen. Griffith likes being special. Apparently, his destiny wasn't just to become King, but to become a god! And here was THE God basically telling him that, since he was also A god, he could create his own destiny!

No, really, do whatever the hell you want! Not only was God telling him all this, but a God speaking for the will of humanity. It was humans who elected Griffith to become one of their five immortal Kings!

Griffith now assumes the dual nature of Savior and Destroyer. It is up to him how to use his powers! Although getting sucked into a God-vortex during a fit of insanity doesn't bode well for his mental state.

Is this the "evilness" that Conrad was referring to? In that case, it's not really evil at all. It is simply the deepest longings of humanity, and the knowledge of how they shape the world. It could even be called wisdom.

"Change this physical field that is your body into a shape suitable for your task." Idea of Evil, Ch. 83

Here we have a much more assertive Griffith. He's ready to OWN this whole choose-your-destiny thing!! His soul body is not disintegrating. On the contrary, it is fully formed... and he wants wings, damnit!! Truly Griffith made a conscious decision to become Femto. But this is a Griffith who has not only went through hell on earth, but also the literal hell of the Idea of Evil. It taught him to stop running away from his pain and embrace it as the driving force in the universe. It became a source of power! His mind had definitely mutated. It was infused with the mass consciousness of the world and now had a transcendent perspective on life. The God Hand were the most accessible, and the most benevolent gods humanity had. It was a lot of responsibility!