Somehow to me it looks like the term "magic king" would fit those guys better than the term "demon king".
They look like human mages that have forsaken their humanity for their magic research and ended up cultivating their magic to the point that they turned themselves into Lichs.
That is what it looks like to me every time I see one of them.
And due to that "Magic King" would fit better in my opinion.
@Talh Actually, isn't Maou essentially Majin (Magic Being) Ou (King)? Demon is just the common translation for it, though I'm not sure if it's exactly magic either.