I need people to remember that bsd is NOT a shonen it's a seinen, it's not black and white or good and evil. Shonens are simple arts that delivers things straightforwardly but seinen puts things in a way that make you stew on it to realise what they were conveying.
The characters aren't 100% good or 100% evil, Dazai? Even though he's TRYING to be good he still resorts to blackmail and torture as shown in the main series and the 55 novel.
Atsushi as well, he's not just an innocent cinnamon roll, he killed Shibusawa and basically went "he deserved it tho" when he remembered. Oh, and remember his fight with Akutagawa on the cargo ship? He punched him in to a burning, drowning ship with no regard to whether Akutagawa died or not.
Another one I always see is Mori and boy do the fic writers love to make him an evil villain that LoVes making others suffer purely for entertainment, like no, he did not make Oda go on a suicide mission to hurt Dazai, he did it because it was the most optimal solution. But There are other better analysis on that so I won't dig into it.
Every character has their own childhoods, trauma and personalities.
Heck, the Organisations as wholes aren't goodie good hero and bad evil villain arggg, the PM keeps the underground in check to protect Yokohama. the ADA, like kunikida says aren't do gooders the go around feeding orphans But they have saved the city countless times. And the special division do what governments do, They uphold the order.
Making the characters one dimensional and one note not only is disrespectful to the original work but makes being in the fandom harder for everyone.
TL;DR bsd isn't so simple, it's a seinen. there's layers to everything in bsd, the characters, organisations, motives, clothes, etc. etc.