this whole post is a spoiler for the whole show in case you're really sure you don't mind that lol...I think I…
I think the explanation lies in the moment when Itaru drinks the poison offered by his father, while smiling. Everything revolves around the symbolism of the poisonous red butterfly that the other harmless swallowtails mimic in appearance, and this serves as the pretext to tell the core relational dynamic that binds all the characters in a cycle of toxic imitation. Every character pursues the gaze of another out of love or for recognition: Anna for Rumi (who did not inherit tetrachromacy and feels invisible, a failure in her mother’s eyes), Rumi for Shiro (who gave her inspiration as a child with his first “kingdom of butterflies”), Shiro for his artist father, from whom he inherited the obsession to preserve beauty at its most perfect moment, though he takes it to an extreme, distorted level, and Itaru for Shiro (crushed between a father obsessed with butterflies and a famous painter grandfather, both possessing a totalizing aesthetic vision). When he sees Anna strike someone with an axe, what triggers in him is not normal human horror. Instead, it’s the artistic logic he learned from Shiro, save the butterfly by killing it, so that its beauty can stop decaying (the boys chosen by Rumi were considered rotten). From there, the leap from human to murderer is inevitable. But unlike Anna, he does not accept being a monster and wants to be stopped. So he chooses to become the final work, the masterpiece his father has always pursued (indeed, for his own display case he paints a “kingdom of butterflies” imitating the one his father once imagined), because if he can no longer return to being human, he can at least become a specimen that remains eternally pure.
Every character pursues the gaze of another out of love or for recognition: Anna for Rumi (who did not inherit tetrachromacy and feels invisible, a failure in her mother’s eyes), Rumi for Shiro (who gave her inspiration as a child with his first “kingdom of butterflies”), Shiro for his artist father, from whom he inherited the obsession to preserve beauty at its most perfect moment, though he takes it to an extreme, distorted level, and Itaru for Shiro (crushed between a father obsessed with butterflies and a famous painter grandfather, both possessing a totalizing aesthetic vision).
When he sees Anna strike someone with an axe, what triggers in him is not normal human horror. Instead, it’s the artistic logic he learned from Shiro, save the butterfly by killing it, so that its beauty can stop decaying (the boys chosen by Rumi were considered rotten).
From there, the leap from human to murderer is inevitable.
But unlike Anna, he does not accept being a monster and wants to be stopped. So he chooses to become the final work, the masterpiece his father has always pursued (indeed, for his own display case he paints a “kingdom of butterflies” imitating the one his father once imagined), because if he can no longer return to being human, he can at least become a specimen that remains eternally pure.