From what my limited MTL and understanding, [spoiler] the teacher misunderstood his relationship with Hitomi (Mano's Sister). This bitch is a real jerk, saying that both Hitomi and the baby she carried is "disgusting". At the day where they got murdered, the teacher came to the house to talk that "he would accept dirty Hitomi and dirty baby" and was sure Hitomi feel the same like him. But Hitomi outwardly rejected him, where it ultimately lead him to kill her ((though i highly doubt her rejection is the catalyst of her murder seeing the teacher already has gloves and knife inside his bag, so this bish surely sick in the head and would probably hurt Hitomi in the future if she accept him))
YESSS! i has the suspicions too that that bitch ass loved Hitomi. Something about him from the start, I always disliked it. Thank u so much i understood it well. But I still don't know who the father of Hitomi is? The adopted brother?? But didn't he just watch from the sidelines in fear when the rape was happening?

Y'all Dan didn't rape the mc's sis. He's the one who led the other guys assault mc's sister inorder to let the mc's adopted brother suffer. Dan was basically psycho crazy about the mc's adopted brother. And I still don't understand why the homeroom teacher killed the family though.