This arc was great y'all just have some awfull takes
The arc was okay-- didn't knock my socks off, but I do think it had value. That said, I'm normally a "let 'im cook" kind of guy once an author's bought enough of my trust.
There's this recurring theme of "Kiku deserved to die slowly and painfully for what she did over 400 or so years" among commenters, but I can't really get into that mode of thinking when it was established that she couldn't be killed outside of suicide because she had no known weakness, and especially not when none of the characters were pursuing her with the notion of killing her for "the greater good". She's not a conventionally moral person, but neither is any vampire, or even Kou. Even Kyouko only hunted vampires because of a personal grudge and a desire to die, herself.
I also don't get the idea that the arc lasted too long. This is the character that was brought up early on as this mysterious, erratic figure others didn't bother with, and she was a big enigma in backstory and motivation even when she had shown up prior to chapter 100. She was connected to another character who was a bit more familiar but was still not fleshed out along with his relation to the protagonist. It makes sense to me for her arc to include an exploration of Kou and Mahiru's friendship as well as Mahiru's past, and for said arc to last longer than other arcs that focused on a particular vampire.
That said, it's odd how she betrays Kou's trust and puts him in a situation where he needs to be bailed out by others, but doesn't hold it against her even a little bit. Was he planning on bringing it up if he discerned Mahiru was being manipulated/brainwashed? Was he planning on bringing it up
after Mahiru got vamp'd?
Why did Kiku die then, because she sucked the blood of someone she loved? And second up, why did mahiro die? If it was because Kiku was his something special from his previous life, then it wasn't even needed for him to have known her as a child.
If that's the case, then yes, it wasn't
needed.
Very little in a story is "needed", outside of conflict. But it's a detail that's important to the relevant characters, and accordingly fleshes them out.
A lot of readers were just here for the slice of life, didn't care nor have the emotional intelligence to absorb the heavier social commentary.
"Heavier social commentary"? There's no need to peacock about how you're supposedly a more astute person for not disliking the arc. I'm not even sure if there was any "social commentary" in this arc, compared to the others.