User blog comment:B214/Volume 23 discussion blog/@comment-201.18.245.30-20170415025340/@comment-29963364-20170415080019

And let's not forget, during the Bandersnach and Jabberwoki attacks in vol.12, not only did the Old Satan Faction attack various locations of the Underworld, but many Reincarnated Devils rebelled and started riots during the chaos.

Most Reincarnated Devils aren't treated as nicely by their masters as Issei (and the others) has been treated by Rias and her family & friends. Issei, and almost every other Reincarnated Devil we've seen in the story so far, are only the "lucky minority" out of a group that is still considered and treated as "low-born trash."

It wouldn't be strange that the final conflict is a Civil War between the Old Generation Devils (who value Blood Purity and Demonic Power) and the New Generation Devils (who value merits and strength regardless of Blood-lines).