Fate against free will

One of the biggest themes throughout Spiral is the argument for fate vs. free will. Almost every character has been told that they have a pre-determined fate that they will be unable to avoid. For The Blade Children they are hunted from childhood based on the knowledge that they are going to have a "switch" flipped in their minds that force them to kill. Ayumu is told that it is his destiny to defy the fate of those Blade Children who can still be saved.

Hizumi on the other hand is told that he will fall into madness the way the man he was created from did. Further, when he loses control, his nearly unparalleled intelligence will force him to release a new horror on the world. Not wanting to live with this fate, Hizumi takes it as a personal challenge. An opportunity to be a hero but within six years he is at his limit. He knows he will die without defying fate to become a hero.

"Though he said I would become a devil like Yaiba it didn't bother me at all. On the contrary I thought I could change that fate."

His actions may make it seem as if he is unable to defy his fate to make his dream of saving The Blade Children come true but a closer look makes it clear that Hizumi was able to defy his fate and have a part in saving The Blade Children even if it wasn't the way he expected to. By sacrificing himself for Ayumu, the one who is 'destined' to be the one to save The Blade Children, Hizumi too defies his fate.

As for the others, Ayumu is able to prove that they make their own destiny by denying his brother's plan. His actions allow Hizumi two more years of life which in turn extends his own life when the time comes. In this way Spiral proves that at least in the face of their characters the power to believe allows you to defy destiny.