It was because of the fact that Red managed to recruit Seiji to his side by having the boy consumed by his emotions and develop feelings of hatred and anger towards Megumi.
Itu karena fakta bahwa Red berhasil merekrut Seiji ke sisinya dengan memiliki anak dikonsumsi oleh emosinya & mengembangkan perasaan kebencian & kemarahan terhadap Megumi.