Demon Slayer: Kimetsu no Yaiba Infinity Castle — Ending Explained

The film concludes with the death of Akaza. After Tanjiro awakens the 'Transparent World' and decapitates him, Akaza initially attempts to regenerate his head, evolving into a grotesque creature that can survive decapitation. However, a vision of his fiancée Koyuki appears, pulling him back from the brink of madness. He realizes his desire for strength was to protect her, and having failed that, his immortality is meaningless. He uses his own technique to destroy his body, dying with a smile as he reunites with his family in the afterlife. Meanwhile, Shinobu is confirmed dead, absorbed by Doma, but the film hints her death was a calculated poisoning trap.

Akaza's Suicide vs. Redemption: Some critics argue Akaza wasn't 'redeemed' but simply quit because he realized he had become what he hated (a bully). Others view it as true redemption through the power of love (Koyuki).
The Castle as Purgatory: The Infinity Castle can be interpreted as a literal Purgatory where characters must confront their past sins (Zenitsu's cowardice, Akaza's regret, Shinobu's anger) before they can 'move on' or survive.