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