User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-4547267-20140716020122/@comment-24098614-20140824005951

My speculation on this is similar because Sirzechs been serving for this amount of time is bad for him because remember they are figureheads in terms of power, and the longer they serve the more people believe in them, and that affects the Bael Clan, and having Issei there would be good for him because he can easily manipulate him into doing whatever he wants, and if Issei is smart he should refuse this offer at all costs because the head of the Bael Clan is trying to use him like a toy.