Why I killed my brother

Killing your twin brother takes a lot. Perhaps sometimes there's good enough reason to do it though. Hanako is a dead boy, who once went by Amane, previously when alive, killed his own twin brother and now must explain to his closest friend, Yashiro on why he committed such a sin. Is it a valid enough reason? Is there even good enough reason to kill family?

(This is based on my thoughts, it isn't canon)