it was all Yuki's inability to be honest that created the "monster". This isn't the first manga where a character chooses to essentially ghost another person who cares about them because the character decides that their existence/ feelings etc. are a burden to the other person,
without ever asking if they were in fact a burden. The other person is essentially blindsided and profoundly hurt, and spends earnest time attempting to understand such immature, selfish behavior. (Not sure why it's so ubiquitous in manga, I hope people in Japan aren't actually suffering through this but perhaps it's common?

) The brothers have no one else but each other, Hayato was essentially raised by Yuki because their mother was unstable and MIA. For Hayato to suddenly be emotionally abandoned in such traumatic circumstances is cruel and would result in acting out.