What draws the line between inhumanity and humanity?

Tsukimonogatari asks Araragi this question and presents to us the beginning of the end for our story with Araragi.

This arc gives the character Yotsugi a spotlight that doesn't really make too much sense until the end. However, her acts of imitating human behavior shows how she's learned to fit in more with humanity even though she wasn't born a human. This is important because the main theme of this story for Araragi is finding out what makes him human in his state that took away his basic physical human qualities.

In the first episode, we get to see how Araragi is trying to change by how he acts as if he isn't worried about Nadeko, but he immediately contradicts this by worrying about the town. He feels as if his departure from his hometown will cause the oddities to roam freely without the fear of him. Even though he knows he can't save everyone, he still desperately tries to hold onto the feeling that he can do it.

In the second episode, we get to see what the main conflict for Araragi is in this arc and it's his struggle to hold onto his humanity. Araragi won't be able to hold onto his humanity if he keeps using his vampirism, but he's been able to save humanity with his vampirism.

Yotsugi voices her concerns of wanting Araragi to remain a human, but deep down Araragi (and we) know that if it comes down to it, he'll always put others before him and use his powers to save everyone. In this arc, we really get to learn that Araragi still holds his beliefs very strong even though he knows that they'll hold value for others more than himself.

The final conflict in this arc challenges Araragi to use his vampirism and risk never being somewhat human again, but Yotsugi takes the bullet for him and puts her nonexistent humanity on the line. She expresses that she doesn't want Araragi to become a monster and to truly appreciate the humanity that he's given that she's isn't as lucky to have. The final scene in this arc with Senjougahara also shows how Araragi's losing the physical features of his humanity will never take away his true human qualities of loving the people around him.

Tsukimonogatari did a great job at showing us how much Araragi has grown, but it also shown us how much he still needs to change. However, can Araragi really change? Is he able to put himself before others and live a life that revolves around his internal world rather than his external world? The question will remain until the end of this final season, but no matter what, all we know is that this is the beginning of the end for the story of Araragi Koyomi.