Think the true horror is the fact that everyone else around him didn't realize he truly thought that way and still believed he wasn't a bad person to the end, that it was all because of his father. This is kinda interesting because the Japanese have that "sins of the father's" takeaway on family matters, but apparently Dazai was so different and far removed from his family name that it made some folks wonder if his father, being a politician and all, drove his son down that kind of path, not taking it into account that was still a choice Dazai made even though he was afraid of telling people "no".
It's "dark" in a sense that it's legit depressing with no real happy ending because a real person lived and died with that kind of mindset, which is a fascinating psychological case study. We joke about the NPC meme all the time, especially when the media freaked out about it as hard as they did, but there's some truth to that. Don't know if we could really label Dazai as an NPC like that, but there was something really wrong with him that no one picked up on, and all we have is his semi-autobiography to go by. So could we categorize him as "no longer human", something akin to an alien or a monster, or was he just such a super broken human that he couldn't personally identify as one?
Had no idea about what the book was about going into the video. I've seen the manga around in Barnes & Noble and just thought it was like a Frankenstein-inspired novel-length story Junji Ito had worked on in between his short stories, though I had forgotten Ito already did his own adaptation of Frankenstein.
Anyway, I liked the video. Watched it in a sitting last night, although the whole time I was going "If Wendigoon thinks this is the most depressing thing he's read, I hope he reads Oyasumi Punpun." Now I'm actually not sure which story makes you want to die more.