A lot of random rants :
Zero's school nurse or Yagari's fiancee was a Level D vampire for X time Remember all Level D that had not being stabilized by their pureblood masters had a period where they keep part of their humanity and control of their urges . Later , under an indeterminated period in the story ; a Level D lost all the remains of his / her humanity / urges . During the time she continues to keep a little of her humanity / self - control , she tried to pass as human . Yagari could had eventired to hide her . But when the time came . She lost the last of her humanity . Turned into a berseker , beast vampire ; AKA . Level E . That time was the same time Zero and Yagari where present with her . She attacked Zero . Yagari could not kill her quickly enough . Because of both his feelings of Zero and him for the girl . Yagari lost his eye and ended killing her in the process .
The fandom definition of Level E is ( from page 135 ) :
Humans who transform into vampires after a pureblood drinks their blood . They're different from common vampires in that they gradually lose their reason and lose control over the urge to drink blood . These who attack humans indiscriminately are called " Level E " .
It's possible to prevent becoming a Level E by drinking a pureblood's blood .
Mega speculation here :
Aido - dono is a very important missing piece to the past related to present Kaname ( the one who was raised a s Yuuki's brother ) , Zero and Shizuka . It's a fact he knew a lot of Kaname's past as the the present Kaname . It's a mystery if he did knew the present Kaname was the same ancestor , Vampire King Kaname from the past . If he knew , Aido - dono had to know about Kaname's plan to kill all the purebloods ; as he did during the time he was Vampire King .
Here's the catch . There are not many people who can be interrogated about the Shizuka incident alive at this moment . Zero knows random bits . Maria knows about the same that Shizuka could know . Takuma did knew a lot of information about the incident because of Kaname confessing him some info . Takuma could also know some details from his grandfather . Rido knows something about this ( but don't expect Rido to talk so soon . ). Of course Kaname knows almost all angles ( if not all ). But the one who apparently got the whole information was Aido - dono .
If Aido - dono indeed was a reliable source of almost all the Shizuka incident events . And he wasn't really being killed . But hide somewhere . He could be a very important clue to clarify what truly happened . But to this to works , He had to be hide by Ruka and Akatsuki . And Ruka has to reveal the truth of her being creating an illusion of killing Aido - dono .
But in this scenario . If Aido - dono is alive . A lot of answers will be found from him .
Of course the opposite could be argue . Kaname killed Aido - dono to prevent the truth out . But Kaname doesn't appear to want to hide his involvement to Shizuka's incident to Zero . So I don't expect Kaname killing Aido - dono just for this purpose .
From the past chapters . Kaname has being told he was involved with the worse sin from a vampire . This sin is " to create a ex - human vampire / Level D without the consent of the human " .
We know that sin was created during the Vampire King period . This implies Kaname was the one who created the sin . Most probably after HW saying to him pureblood vampires needed only a few servants for keeping them alive . Humans who consentedto be vampires .
So if the sin was to turn Zero vampire . When we know Shizuka was the one who bites Zero ( and the one who commited the sin ) . Why Kaname is accused from having a participation or being a witness here ? A reason could be he was the intellectual author . But this is questionable as he had told a half lie / half truth to Zero about his involveent .
Kaname could be also an indirect participant of this .( waht I'm more inclined to think ) . I believe after Kaname knew about the existance of Shizuka . Her being the fiancee of Rido . Knowing about her hate for the Vampire Senate and Rido being encaged . Kaname could plan to free Shizuka . Believing she would expend her revenge over Rido and / or the vampire Senate . ( Remember Kaname couldn't kill Rido himself . Because of the bond between them . ) And his plan backfired . Due so somehow Rido and the Vampire Senate discover her escape with a ex - human lover . Altering the vampire hunter's list . Changing Shizuka's goals of vengance .
This issue is one of the issues I'm getting a lot of troube getting a workable theory . For now what I know I got rigth was : Kaname's being the one involved with Shizuka's escape . ( This one from the very Shizuka mini arc . But no many comments at forums from me about this .
So now the thing I'm sure it will happen is someone at last narrate the whole incident . To make clear what happened in reality . Whose this person will be ; is kind of a mystery to me yet . As I said Aido - dono is the best witness for this . But it could be any other person .