The conclusion of this arc was interesting. I feel like this is one of those where the journey was more important in the actual results. The two big things were
- Koyomi Araragi in seeing how much he has grown through First Season, Second Season & Final Season. But at the same time as he is still a child as Senjougahara mentions. Implying he has more growing up to do.
- The other being the relationship between Shinobu & Suicide-Master. How things have changed since their first meeting to now.
The first thing being we had our meeting between Kie Harimaze & Suicide-Master. As we learn that Kie was the one that got turned into a vampire and not Sowa Kiseki. The body swap is a clear reason why the information was wrong because the only way that they can identify a mummy is through belongings.
Now the interesting bit that Suicide-Master mentions is that Kie pleaded to be a vampire. We just assumed that Suicide-Master ate Kie for her own sake. There is a lot of Kie that we don't know much about. What was her life like? Why did she want to turn into a vampire? Given how she attacked her teammate, I can only guess that there was some conflict which is attributed with both Kanbaru & Higasa no longer being around. It feels like the lack of leadership caused Kie to go down this path. It does also bring an interesting parallel to before Shinobu turned into a vampire. She did because if someone had to die, it would be better for it to serve a purpose rather being a meaningless death, which caused many people to die when coming in contact with the Beautiful Princess. For Kie it seems like her wanting to be a vampire was for her own self and nothing more.
As Suicide-Master put it, she fell in to despair. This feels like a scenario where someone puts their whole life for revenge and once they get revenge, there is this giant empty feeling. I am sure she would have gone after more of her teammates and the cycle would have repeated. At the very least, becoming human again could allow her to tackle the underlying issue, whatever it may be. As a vampire and the power she has, she wouldn't face the issue head on.
Plus being eaten is what Suicide-Master would have wanted. She lived a long time & after seeing Shinobu and how happy she is, there is nothing more she needs to do. No reason to continue living when you have done all that you sought out and death will be coming for her soon. Though for Kie she wanted to continue down this dark path. It seems it even deluded her that she was saving her teammates. Which I doubt was the case, but sometimes when you are so far gone you will trick yourself into thinking what you are doing serves a purpose.
As Kagenui shows up, we got a little banter between her & Suicide-Master. Really showing how weak Suicide-Master has become where Kagenui doesn't think much of her. Suicide-Master lives since it is clear Kie was the one that caused this and the solution was to turn Kie back to a human, so the other can turn human again.
Shinobu's comment that humans are monsters is something that may be more true than you think about, You think about how Kie wanted to be a vampire and what she did when she got that power. Also, I imagine Shinobu seeing all her subjects killing themselves in the honor of the Beautiful Princess's soul is something they aren't worth. Through her whole life before meeting Koyomi Araragi the person she was closest to was Suicide-Master. A vampire was the person she held the deepest of feelings for and felt like family. Makes you think perhaps humans are the real monsters.
It seems implied that both Senjougahara & Shinobu knew this was the final time they would see Suicide-Master. This is where Koyomi still feels like a kid despite the major growth he has gone through. Koyomi thinks of as a farewell means this is a temporary goodbye, and we will see them again. In reality, the world is more cruel. Considering Suicide-Master just recovered from being a vampire doesn't mean death isn't immediate for her. She will most likely enjoy what time she has left, but knows her time is up. After all her request to be eaten is proof of that.
The other thing is what Kagenui said that she doesn't hesitate because she is an adult. I think what Kagenui implied is that she does what she needs to do. She has an understanding of her actions and the consequences of them. For Koyomi there is that hesitation that he thinks about Kie's situation, for example, before passing judgement. On the surface, it feels like Kagenui doesn't care why Kie did all this. It is more so she is aware that Kie might have circumstances, but those shouldn't have an effect on her decision. She will take responsibility if there was an error on her part. For Koyomi he is an incredibly kind person, and thus he tries to empathize with others. His arc with Sodachi in final season is one big example of this.
One thing I wonder is this really something Koyomi needs to grow out of? His desire to help out others when it is needed? I don't think there is a clear answer for this because his kindness is one of his strongest traits. It is that kindness that resulted in Hitagi Senjougahara falling in love with him. I think there are some things Koyomi could learn from Izuko Gaen. Delegating and understanding when he needs to get involved.