Responses
http://www.mangago.zone/read-manga/makkurayami_de_kimi_to/mf/c005.5/10/
This character is actually his father. It is par from the main story http://www.mangago.zone/read-manga/makkurayami_de_kimi_to/
his father’s parent was always beating his father so his father learned that way. His father cared about his sons so he hide him in a box that way he can protect him from himself.
has done everything to make me super curious about their father, but has left me with as many answers as the sons.
If i wasn't mistaking the flashback, the father did care for both sons, but maybe Yuu's resemblance to himself made him feel uncomfortable around him. Maybe he was scared Yuu'd become like him (broken and twisted).