Honestly, I’m not sure his quotes show a linear development so much as they expose the raw, unchanging wound at his core. Everyone points to his final conversation with Tanjiro as 'growth,' but re-reading his early taunts to Rengoku, the same themes are already there—this obsessive, nihilistic fixation on strength and the emptiness of those who lack it. The words get softer at the end, but the core belief that only the powerful deserve to exist? That never actually changes. His backstory monologue just explains why he’s stuck there, frozen in the moment he lost everything. The quotes feel less like progression and more like peeling back layers to show the same rotten foundation.
Maybe that’s the point, though. For a demon who can’t remember his human name, development was never really possible. His most famous lines, 'Why can’t you become a demon?' or his rage at 'wasting' his strength, they all circle the same drain. The tragedy is in the repetition. He’s a broken record of grief and arrogance, and even his final, quieter moments feel like a sad echo, not an evolution.
I actually find that more compelling than a neat redemption arc. It’s a portrait of someone so thoroughly consumed by a single idea that they can’t move forward, only dig deeper into the same hole.