He didn't die because Sangwoo stabbed him. He didn't die because he hit his head. He was just runned over by a car asdfgh. After that, they finally bury his body (chapter 21, teen Sangwoo is seen carrying a man aka his dad covered by a jacket).

What did Sangwoo's father do to him?

He was once a apparently hard working, kind man who cared for his wife and son, until due to unknown reasons, he became abusive and frequently abused his wife, even keeping her tied up in the basement without telling his son where she was who never noticed that his mom was gone for days while she was being locked in ...

Who killed Sangwoo's mother?

she died from sangwoo indirectly stabbing her where she impaled herself on the knife in her throat. (in ch60) in today's chapter, lee tells seungbae that the autopsy showed she died of hemorragic shock, which is basically just extreme blood loss.