I think his dad is abusive or somewhat bad so he didn't want the marriage of his dad and D...

I think his dad is abusive or somewhat bad so he didn't want the marriage of his dad and Dajeong's mom.That must be why he strangled him.So can let it silde.but from the main story,the way he imagine Hyeok while fuc*king Dajeong is a no for whatever reason (╬￣皿￣)凸