It might have begun with Go Nagai's series Devilman, which evolved from a supernatural superhero story into a full-blown apocalypse with the hero fighting an ultimate battle with Satan himself, who's the big bad of the series He lost, humanity is exterminated, and Satan, who actually loves him, is left alone with his bisected corpse[...]
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This is also influenced by the aftermath of a climactic battle near the end of Go Nagai's Devilman manga and its faithful anime adaptation: the hero is left grief-stricken and carrying around the severed head of his love Attack on Titan subverts and reverts that image by having the heroine, who was always the better[...]