That he had his memories blocked, he had an idolized image of her but in his flashbacks, she always appear backwards or with the face all blacked but in the flashback of the "novelist" you can see shark teeths so her appareance was different.
Since the beginning when the wife abuser say she was coming for revenge and he didn't understand it was obvious that he was involved in her death and he block his memories so he was the killer.
I mean you were 7 fuckin' morons in the school, there's no way you won't know she was being bullied.
Yet I don't see why she hate Mizuho as far as has been shown in the flashbacks she didn't bullied Kiriko.