That's not exactly how they see it.
Blood lines are important because the blood lines are believed to be the 'spawn' of the various Nephilim, each of whom descended as subdivisions of Lucifer.
They believe their own spirits are a heavily fragmented form of Lucifer, and that they have been empowered to have authority over human affairs with duty to subject and test humanity according to an imbued sense of higher moral reasoning. In short "whatever heinous thing I can come up with is a test for humanity and that makes it a moral challenge for them."
Of course, one must wonder why they endeavor so hard toward the evil, as opposed to a more good moral alignment, but that is where the plan comes into play. Date A Live explains that one.