These would be reasons why Naberius Narnia is seen as a villain:

While under his Fenrir guise, Narnia is revealed to be a deeply racist person who seeks out a Netherworld for demons only and eliminates all other species that inhabit it, such as deities or magical beasts. Narnia, however, has a much deeper hatred for humans in particular. In fact, he loathes Henri's policy of sending strays back to their world, believing they should be eliminated on sight. Narnia perceives all these other creatures as invaders who bring chaos and disrupt what he believes to be the "balance of the Underworld". Narnia's main motivation was to secure a place among the 13 crowns in order to gain the political power to make his Puritan visions a reality.

I would also add the fact that Narnia recommended Atori and Shida, who are actually 2 of the 6 members of the terrorist organization the 6 fingers, to teach as teachers at Babyls (I don't remember the chapter from the manga in which Opera san, specify this thing, but I think it could be about chapter 275).

In chapter 275 of the manga, from the Post Heartbreaker arc, Naberius Narnia recommended Atori and Shida as teachers at Babyls. Instead, Narnia received Baal's recommendation as a member of the 13 crowns.

In chapter 306 of the Post Deviculum arc, we learn how Baal congratulates Narnia for becoming a member of the 13 crowns. And Naberius Narnia saying indirectly referring to Baal let's continue to use each other for our own personal gain.

In chapter 305 of the manga, when Kalego asks his older brother Narnia why he recommended Atori, Narnia answers that it was necessary. Narnia also tells Kalego, regarding Atori, how she knew it would cause him problems. Which would mean that Narnia knows how Baal has connections with the 6 fingers, but does nothing about it.