Well, I am speaking from my own experience, and relating Dostoevsky's works with my own life, and I probably shouldn't have said "thinking nihilistically" but rather losing hope. For instance, if you have read C&P, Raskolnikov does the murder in order to achieve something great, something above himself, something that will raise him above his current position. However, after he commits the crime, he realizes that what he did was for nothing, that everything is in fact the way it was before. That is what I mean by being nihilistic, that feeling that whatever you do, in the end it doesn't matter, in the end nothing changes. In C&P, Raskolnikov found hope in God and love. Notes From the Underground, in my opinion, presents what would happen if a person cannot find hope and solace in anything. The Undergound Man has lost all hope. It would be a continuation of C&P if he did not find love or God. Our hope, of course, is to be perfect, to always do everything right. However, the Idiot portrays what happens when you become too perfect. It is like a continuation of C&P if Raskolnikov does become perfect in every way. I hope I can be understood, but if not, I'll try to explain even more. I don't know, I hope I don't sound too crazy.