I'm confused as to why the boyfriend wanted to kill the girl and her family. The flashback of him when he was younger confused me. What happened back then? What did the dad put in the man's pocket? And what happened to the boyfriend that would make him want revenge on his girlfriend's family?
shareI agree frost1013. After watching the show, I became highly confused by all the randomness. My personal thoughts are this: The family appears to have a curse on them that will allows them to wish people dead. Remember when she was sitting at the table at her home alone and she asks her father if he ever just wanted everyone around him to vanish and die? Well her wish was coming true. The boy in the end that says something like I can help you, may be a demon of some kind that feeds off wishes of harm to others.
I did some more research and found out that there was a comic book which goes further into detail of the story line. The family is indeed cursed because a long time ago, they killed a giant magical snake to bring them luck and it's last words were to curse the family to make someone in the family kill someone every generation or something. The film didn't add this for various reasons to which I can't explain. I think it would have made the plot a little more clear some of us.
Not sure if the OP will get this but at least this can help others. As I took it, the boyfriends dad was in financial trouble. It is possible that he worked for Ga-in's father as her father made a statement that led me to think that. Anyway, his dad simply gave him some money as though he was a common beggar. Ashamed he decided to kill himself and his family. His son lives and thus seeks revenge against Ga-in's family. That is why he got close to her.
As far as the over all story I think you have really TWO stories going on. There is Ga-in's boyfriend who wants her family to pay and thus kills the mom and dad and plans on killing Ga-in. There is also the brooding student who warns Ga-in to trust no one. He appears to be a demon. A living form of evil. Satan you could say. I think the idea is he does not make people do evil, he just puts the weapon in their hand and gives them a little nudge. Then they do what is their true desire, what is deep in their heart.