A crazy theory.
Maybe Naberius Knightwalker instilled all his ideologies in Narnia.
But what if someone else killed Naberius Knightwalker because he represented a future threat to the future demon king.
And what if Naberius Knightwalker also killed the interim demon king Kushinashi. Only if Kushinashi had sensed that someone from the current crowns would plan to kill her. And then Kushinashi asking for the help of a former member of the 13 crowns from Derkila's time to help her with something.
After Knightwalker kills Kushinashi, Knightwalker is killed by someone else, and that someone else could be Uncle Naberius himself.
In a chapter of the manga (I don't remember what number the chapter was) , we are presented with almost all the former crowns from the time when Derkila was the demon king and on the cover of volume 1 the former members of the 13 crowns are represented as well. On the cover of volume 1 there also appears a bird and the magic of the Naberius clan bloodline, namely Cerberus. When Iruma came to the Barbatos clan estate accompanied by Robin riding a bird, it is possible that the chairman/chief of the Barbatos clan when he was young was a member of the 13 crowns.
And the Cerberus on the cover of volume 1 does not coincide with the Cerberus of Knightwalker, so most likely the Cerberus in question it was Uncle Naberius.
In other words, when Derkila was the demon king, Uncle Naberius was part of the 13 crowns, but after Derkila left, all the former crowns/almost all the former crowns were divided. And in place of Uncle Naberius as a member of the 13 crowns came his older brother Naberius Knightwalker.
And maybe Uncle Naberius told Narnia that his father became interested in an imaginary creature (humans) and that's why he left when in fact Uncle Naberius killed his brother because he would represent a big threat in the future. And the reason for this lie would be so that Uncle Naberius would not be hated by his grandchildren.
And like the butterfly effect, Narnia came to hate humans, most likely starting from a small lie.