There's something horribly fitting about Kanda's body crumbling as if he was made of porcelain, with cracks spidering over his torso. That visual metaphor of Kanda as a doll, Kanda as an artificial person, Kanda as a construct, Kanda bounded by the externally imposed limits of his crafted body. It's this terrible visual reminder that Kanda is not a person, at least as some of the upper echelons of the Black Order are concerned. He is a tool crafted for their use. So is Alma. It's that perspective on the Second Exorcists that ultimately leads to the Second Exorcist project's downfall.
ETA: The juxtaposition of shattered-doll Kanda and the current arc is even more striking when you consider that this arc has been Kanda's backstory, clearing up the mysteries (like where that tattoo of his came from). It's been the process of fleshing him out and giving him...interiority, I guess. Allen (and consequently the reader) even get a chance to literally dive into his mind and his memories and his past. The cracked-doll imagery of Kanda's death, however temporary it may have been, is doubly terrible when considered next to that; it's a reduction of Kanda to an object just as the reader's been given all of this information that makes Kanda a person.
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