User blog comment:B214/Question and Answers 6/@comment-187.180.176.223-20190619153726/@comment-10330822-20190710002652

He states that when he talks to Issei about trying to encourage Issei to "run" for Satan.

You remember that conversation, correct? Basically Zekram Bael doesn't care about the lower pillars, the Bael Clan is all he really cares about, because its his house. So if Rias marries Issei, it makes no difference to him. What he wanted was for Issei to become a Satan because he felt if Issei did it, then Sairaorg would be compelled to do so as well, which benefits Zekram because he does not approve of Sairaorg as the next head of Bael. If Sairaorg became a Satan, he'd have to disinherit himself from being the next head of Bael.

That was why he invited them there, to try and convince Issei to work with him, saying he's support his bid to become a Satan if Issei wanted the position. When Issei questioned why he wants Sairaorg to become a Satan if he doesn't approve of his as Bael Clan Head, its because Zekram points at the blood purity is everything, because despite the Four Great Satans holding power on the surface, the devils only recognize the blood.

So, that is why Zekram states that the Bael Clan is the true leader of the Underworld, because if it came down to it, Zekram implies the Underworld would support Bael over the Satans. The Satans are just pieces controlling the system that is in reality controlled by the Bael. So, in Zekram's eyes, its less acceptable to have an heir that does not have the suitable demonic power, even if he is a pureblood, because in his eyes, there is no greater authority than the Bael Clan Head, than it is to have Sairaorg as a Satan. So to him, Bael Head > Satan.

Does that make sense? That is how the purebloods really think, and its a very dated view, because as Sairaorg proved, you can surpass someone with demonic power through training alone. In a way, when Sairaorg beats Magdaran in their duel, he threw the purebloods into disarray. This was even further complicated when Issei then defeated Sairaorg as a reincarnated devil.

This is presumably why Lord Bael, not Zekram, Sairaorg's father, hate's Issei so much, because is literally the poster boy for the Anti-Purity Faction that Sirzechs and Ajuka seem to be pushing. To be honest, I believe this is being done on purpose by Sirzechs and Ajuka, but that's another conversation.