||Original idea?||
Dazai live in a traditional family he have almost everything except one thing warm of a family his father is very distant and doesn't know how to show affection meanwhile his mother doesn't even cared for him she always avoid him the moment he turned two years old she keeps going out and coming home late since he was the only heir at the age of three he have to study non stop his schedule is always about studying he did not have time to enjoy his childhood because of lack of communication between him and his parents he doesn't know how to communicate or socialist with other people in fact the moment he turned three year old he feel like he doesn't belong among them that he is somehow different he became afraid of humans because of there nature the first one they're sweet and kind but the moment you turn your back or if they become angry they will show they're true nature that was one of the reasons he didn't ask for help when one of the maid assaulted him not even a single person knew of it later when he turned eight years old his mother dead because of an "illness" that's what they said to dazai but he knew the truth about his mother death his father who have many enemies killed his wife aka dazai mother so to protect dazai he entrust him to Mori but he didn't know it will be his biggest mistake because he didn't know that Mori was working under the port mafia one of his enemies it was to late for him when he realized it they used dazai as a hostage to make his father back down because they have dazai Deon doesn't know what to do so he was constantly under pressure because of his weak health he fall ill making him an easy target to kill one night Mori planned something to make dazai under control it was to kill his very own father they force they way in other pm member killng unimportant people and Mori and dazai going into his father room when they get in the room Mori force dazai to murder his father although dazai doesn't feel any affection for his father he didn't want to kill him but Mori still force him He gave him a knife and grabbed Dazai's hand and force him to kill his father with that they or he(Mori) stab deon four times after that Mori released his hand on him and walk out for a minute to check out the others dazai father Last words made dazai feel guilty for the rest of his life.
[After that everything in the anime is the same I hope it makes sense]
you can use it if you want has long as you give credit:)