Another commenter explained it; it's a first person pronoun that gets kind of lost in translation. In Korean (as well as Chinese and, to a minor extent, even Japanese iirc) there are ways to refer to yourself that reveal your social status. In this case, the pronoun used by Beelzebub is one that hints at the status of a King... but in English we don't really have that. Here it gets translated to "we" as a way to keep the mystique of this character being a ~strange demon~ or something, I'd imagine.
If it got translated to smth like "I, the King," then it would no longer be a pronoun and not only it's usage would be too excessive in the long run (think about how many times we say "I" when talking), but it could also come off as him tooting his own horn, when that pronoun usage in Korean is still taken as dignified.
On chapter 14 and this man keeps talking like he is multiple people is weirdin me out