Kekkaishi e no Tensei - Vol. 3 Ch. 15

[MangaDex]

 

[Goldni]

wait hes the demonking now i feel like i missed reading that part

 

[Bloddrake]

wait hes the demonking now i feel like i missed reading that part

I think he became the Demon king for killing every human in the castle and the accumulated grudges and malice from that action, that was the reason why the villagers called him a monster and couldn't see who he was.

 

[Quesocucuy]

lol that last panel
bro likes 'em T H I C C

 

[Feroen]

I think he became the Demon king for killing every human in the castle and the accumulated grudges and malice from that action, that was the reason why the villagers called him a monster and couldn't see who he was.

I don't think he is explicitly a Demon King, just that one was assumed to have caused the incident, since he claimed demons descended upon the capital.
Edit: No he's definitely apparently an "Ex"-Great Demon King sooooo shrug

 

[icam98]

lol that last panel
bro likes 'em T H I C C

Bro knows whats up

 

[Thunderator]

I read a "spoiler" comment from a couple of chapters ago that said he was a demon king and that's literally the only reason why I know of it. But then they just tell him about that and he acts as if he already knew?? It really does feel like we missed something, maybe during the translation or the author just said "fuck you" to us