it was shown that only by the beginning of the second arc did Kaname order Ruka and Kain to check the Hanadagi's, in the first arc none of this was shown, and we know where Ruka and Kain were. Therefore, only during the time around the second arc did Kaname begin to monitor the Hanadagi's, for whatever reason. Hence, the premise "anything that disturbs the castle will prompt Kaname to move"
is not too accurate, since anything can disturb the castle in the first arc, and would necessitate surveillance, but Kaname didn't do that.
Therefore, something in the second arc prompted Kaname to watch over the Hanadagi's. Knowing his character, he had probably foreseen the events that will happen in the castle in the future, due to something. This something can be anything but let's see the new events that ignited at the start of the first arc:
-the kidnappings of the children by vampires under orders from a "master" (ch49,last)
-Sara kidnapped Takuma before 2nd arc began (ch45,p28) and Kaname knew where he was and told Rima and Shiki not to search from him (ch53,p29). He sent invitations to both Sara and Takuma for the Kuran ball (ch52,p28). Then during the ball, Kaname watched over the room of Sara's fiance, Ouri's, thru Seiren, perhaps foreseeing what will happen(ch54,p28). Something did happen and Kaname knew it, but he let Sara get away(ch55,p23-24).
before the ball, Kaname already told Kain and Ruka to watch over the castle (ch52,p19-20). I presume it was the first time they went there, since it immediately alerted the castle's guardian. Hence, their mission was a new one, and recent. this was in chapter 52--- 5 or more chapters after
Sara kidnapped Takuma.
So is it a coincidence, or Kaname appears to have already foreseen what Sara could be after by chapter 52 just because she had kidnapped Takuma? the other steps Kaname made after that seemed like they were just to confirm his suspicion on Sara. But again, he made no move to catch her and even allowed her to do what she wants until chapter 67.
But notice along the way to that chapter, there were anonymous intels that the hunters received pointing out to Sara's new movements, the harem (ch58,p22) as well as tablets (ch65,p9). But again, the hunters did nothing about this, or they stopped by the time she made her move on the tablets. And hunter-san merely "watched" (ch65,p9). The hunters did nothing
again when Sara comes to their doorstep.
There's a deliberate
attempt to ALLOW her to get away with everything, both from Kaname's part and the hunters (or just hunter-san). I can at least safely say that this is confirmed.
So who decided she should get away? The one who planned all this from the beginning, Kaname.
Why let her get away? thats the question... Sara had been getting her way since she started her plans. Kaname even "cleaned" up after her by taking the blame of the mess she made and becoming a murderer... But the fact is, kaname had been watching over Hanadagi since the beginning of Sara's plan (kidnap takuma) so he had planned to take the blame all along. Even perhaps, to be branded as a "murderer".
Now, looking at Sara's actions in the castle..
Sara went in the castle thru Ouri's power and nearly killed the guardian. BUT she doesn't finish her, or that's what was seen (ch67,p18). That's one witness.
Sara then thought she might need blood to awaken Hanadagi and if so Aido-dono would serve the purpose as a "snack". (ch67,p21)
Aido-dono then warns her not to start another "conflict between purebloods" and Sara replies with "If you're scared then go and hide underground" (ch67,p22)*Note the "conflict within purebloods" and her suggestion to hide "underground."
Then she rips out Hanadagi's heart. And then she flees, leaving 2-3 witnesses behind, and alive! If she wanted to be clean to the public's eyes, she would not let them live. But first of all, her priority in the castle is to take Hanadagi's heart, and second she is "weak", and she needed a hunters hand to kill Hanadagi(ch67,p23). That alone is enough reason to have no choice but to leave the witnesses be. Therefore, there's NO point killing all witnesses when there's at least one witness left alive that she cant kill.
Now, about Kaname's participation in Sara's plans...
Note that Nagamichi had mentioned before "the conflict between purebloods" and warned Sara not to start it. The way he phrased it sounded like she was going to start a war between purebloods
, and it also matches that Sara said, "Yes, many lives will be lost". She also says, "If you're scared, you can hide underground
until its over". She knew there was going to be a huge massacre happening, so yes indeed it bears similarities to a war.
How she was going to start it? For one, she took the heart of the "protector of the purebloods" as his guardian said (ch52,p20). This Hanadagi person must be considered the VIP of the purebloods: he had one guardian who guards his castle, PLUS a seal in his castle that can only be broken with Ouri's power. Ouri, as told by Hanadagi, was also a "powerful" pureblood, which surprised him that a "weak" girl like Sara managed to take his power (ch67,p24).
Therefore, two fields protect Hanadagi: one was the guardian, and the other was the powerful seal of Ouri. *Ouri, I presume, cast a spell or a barrier on the castle which was rendered useless with his death. OR his power was the key itself for that seal to be unlocked.
So yes, Hanadagi is theoretically an important figure in pureblood society. It can even be speculated that he is the symbolism of the thing that holds the purebloods at bay. Or something to that effect. *Note how hanadagi wanted Aido-dono's blood to "know what was happening in pureblood society". In short he wants to be updated to the latest news... for what reason? He just got his heart ripped off and he wants to know the news?
So there, it is hinting on the importance of Hanadagi's role... What would be the meaning of his awakening?
*Note how Nagamichi recognizes his presence and even bows reverently towards him upon his rising. And Nagamichi seems to know his history lessons as well as what just happened with Ouri's castle-- he knew there was a seal on the castle, he knew what was needed to break the seal and he also knew what it means to break that seal.
Why Kaname killed Hanadagi for Sara? He had brought the cursed hunter's sword with him knowing he will use, he had the castle be watched over for some time, and he waited for Sara to make her move. All this so Sara could get her way.
Now, if we piece the clues so far...
Sara starting something like a war between purebloods by killing purebloods...
the massive conflict will arise due to that...
Kaname killing the only witnesses to what had happened...
could it be that Kaname was preventing that information from leaking to the public? So he killed all the witnesses and let himself take the blame?
If that information leaked to the public, Sara would be the criminal and this would stunt whatever she is planning as the hunters go after her, which does not fit according to what Kaname allowed for her to happen. He WANTS her to get away with her plans, and one thing to remember is that her plans also works for his plans.
Another possibility is that Kaname might know the panic and chaos that would ensue when the information gets out in public...especially among purebloods. If the purebloods knew Sara just started a war between purebloods, they would be picking sides and start eating each other, or to that effect. Hence whatever she did MUST be kept a secret
*This must be related to the subject Aido-dono and Takuma discussed in separate chapters in 2nd arc (ch65) (ch66): "The greatest crime a pureblood can commit."
*Remember the time when Kaname killed Shizuka out of need and took her power? And he placed the blame on Zero? Kaname had just killed a pureblood, and he didn't take the blame for it, saying "he still have some things to do at the academy.". Remember the fact he doesn't owe up to this crime, BUT he owed up to the fact he had wiped out the council members. He let Zero take the blame, though unintentionally, and then bailed him out later by defending him.
*As for Zero and Yuki killing Rido, they didn't eat his heart. or take his power. He simply died.
*This "secrecy" might be connected to the letter with the wax seal Isaya received in chapter 69, if the contents of the letter informed him of what TRULY happened in Hanadagi's castle as well as the hunter's involvement, then it was an information that was exclusively for HIM, and perhaps it also told him that he was "safe" from being killed. Hence, the wax seal was used to authenticate the contents of the letter, and to prevent any unauthorized person reading it.
Sara got away with it, and she knew someone covered her up. She would be stupid if she told anyone that she just started a war or ate purebloods' hearts, so the best way to make up for the situation was to PIN the blame on Kaname, effectively diverting suspicion away from her. It worked for her plans, and I think that's all that mattered for her.
Kaname probably also knew that Sara was a sucker for anything that "worked" for her plans, he makes use of this and he allows her to get away and allow himself to take the blame.
Kaname then, being the fugitive with a dirty name, takes advantage of the situation he is in OR he goes according to what he had envisioned all along. He kills all the heads of the pureblood families, for whatever reason I can't think of yet, and activates the volcanic contraption that will "start a climate change".
My speculation is this, he wants to prevent the 'war' from getting worse or from officially starting or going public. So he kills the pureblood heads, only the heads of the families, before they found out the truth that the war already started, to prevent their participation in it by aiming for the "head". Then he activates the volcano, perhaps, to serve as a diversion in case the war does
officially start, buying him enough time to do whatever he plans. Or it can also serve to "cleanse" the place while he was starting his reformation plans for VS.
As we see Sara seems very confident now and has completely let her guard down as she resides inside the academy, having made no new move so far. And this is probably what Kaname was expecting: she would eventually become idle as she gets what she wants.. But not yet. She started working under the shadows thru the tablets. Just as she does, Kaname appears near the area, and as he does he also seems to make a visit to Isaya's house... Perhaps to visit him officially?
Yuki as a distraction for Kaname is duly so. Its just like all the romance movies where the guy doesn't want his woman hurt or involved in danger, so he leaves her forcibly to her foster father.
Kaien as the one who holds her back is doing an effective job, she was given duties as if she was just living a farm life but with a lame excuse. Though as for Sara being on the same grounds as her, and Kaien doesn't know that nor does Kaname seem to mind it, maybe it will be revealed soon.