I can't remember my source, but I remember reading somewhere that Jubei told Ragna that the cauldrons at the NOL could potentially form another Black Beast, and that's the reason he goes after them....Aside from trying to find Termui, I guess. And I doubt that the NOL oppressing people has anything to do with Ragna's attacks. He doesn't seem to give a damn about what they do to the people.
Of course, I could just be remembering wrong about all of this. I'll just go sit in the corner now.