and with this, I too become an offering...

“It's said that the book he accepted is bound in human skin.
It was composed of curses towards humanity in every conceivable way.”
This is true. Also worth mentioning is the spellbook also being referred to as “Textbook of the Sunken Spiral Castle”.

”By rights, the book deserves to be burned.
However.
As he is now, it was the truth of the world.”
The only thing he knows is the book and the curses it brings, and the fact he knows it's wrong is where the conflict arises. It really shows how manipulative Prelati was, coming in when Gilles was most vulnerable with something he could believe in and devote himself too to replace the part of his life Jeanne fulfilled.

"You have crushed my saint."
He screamed to the heavens.
"Therefore, I too shall trample the miracles you have created.
I shall slaughter those pure and lovable children to my heart's content."”
He views the brilliance of Jeanne and the joy she inspires to be the same as children, hence the killing them. One of his dialogue lines for something he likes is “I love children. Their smiles really brighten my day.” Really feels like a punch in the gut when you see how he’s ended up.

”The man's pitch black eyes were utterly stagnant…”
There’s a subtle difference to how his eyes are depicted, looking completely black as a saber but having a hollow shine to them as a caster. That makes it seem like this monologue was, I guess, his last revolt as a sane man. Pretty neat.