Surprisingly Complex: M.S.S.S Episode 13
The episode starts with a conversation with Nadeko and the Serpent. We can clearly see the Serpent is a liar. We can see that there is another reason for the nightly stroll than just searching for the body (Which, if you ask me, doesn't exist). The oddity has only taken the form of a Serpent to further it's cause. If we stick with the idea, that the Serpent has born out of doubt, it taking the form of a snake shouldn't surprise. You see, the part where Nadeko can't run from responsibility is from killing all those snakes. That was the only time she felt she was not a victim.
I'm not sure how much Araragi saw through Nadeko's smokescreen, which was actually more like vapor. But it doesn't matter, because Shinobu saw right through it, just like the watcher should have. I'm sorry that I'll bring it up again, but the theory is sound once again. Nadeko always plays a victim, but the fun part is in the part "play." What is a person who plays a victim? A FAKE victim. Yes, that one again. Nadeko can lie to anyone just to make sure that she isn't suspected of being guilty of anything. And with her, is a FAKE god. More fake. But, arguably, the Serpent being a fake god, it makes Nadeko really easily manipulated. She even aknowledges that the Serpent is a liar, but in the fear that taking an active part in the discussion would make her seem like a hypocrite, she cannot question the Serpent.
So, when Shinobu asked a question, why did Nadeko answer: "Evil?" The question was about the type of person Nadeko was. Her black-and-white way of seeing it made her absolutely think that in the first moment that she is doubted, she must be evil. She thought this, because in her mind there is either a victim or a prosecutor. When Ougi activated the train of thought that ended in the shattering of that idea, she automatically makes everything in herself seem evil.
So, what part does the Serpent take? It's there to make Nadeko doubt more, to make her see herself more and more evil, which powers the Serpent, because to see oneself as evil means doubting your very intentions, manners and ways of living. The fact that the Serpent didn't answer during the brief moment Araragi was gone was precisely this. She must be made doubt, not only her self, but her sanity as well. Okay, that might be a stretch but I'd say that I'm on the right tracks in here. Not sure how much exactly, but somewhere there lies the truth of the matter. We'll see it in a few episodes.
Nadeko's self-doubt of being evil makes the fake Serpent strong.