Dai-bosatsu tôge

There is, of course, no question that the protagonist here is the handsome and insane Ryunosuke. And I can see that his insanity, alienation, and disassociation all come together in a climax of hallucination and compulsion. But there are things that I do not see. First, why is he insane? That is, what is his origin and the origin of his evil style of sword play? Second, why is there at least one very absorbing subplot which is developed and then dropped? I mean the romance between the "grand daughter" and Mifune's chief student and the tension regarding their welfare that builds up because of the plan of revenge? Is there some overriding master plan here; or some historical, cultural, etc. concept that I am missing?