I have several responses.
I still find hard to understand perfectly the Japanese xD
But Torigoya transforms Acchan allegedly mentally that we see, Acchan is very strong physically and apparently weak point are the memories.
Now I think Yuko Sado die and not the slap by jealousy but by the sadness of knowing the trajedias and apparently in love with her.
Moreover all Acchan accuse Minami's death to the mysterious girl Erena.
But flash back to pass suggest that Minami and Acchan estubieron in a fight very hard and could not defend Acchan Minami so multiple Minami beat to death Oo
Well that's what more or less understand.
If someone can give me a better answer lo agradeceria, to understand more this great dorama!

That certainly is a great place!
