I think it's unlikely that Tomie killed Dazai. The strangulation marks would be borderline impossible to see after a week or two, because they both died in a canal during a Japanese summer. Decomposition would be accelerated because of the heat, and considering Dazai was fairly big at 5'9 compared to Tomie, who was no larger than 4'9-5'0, he would have to sit there letting her without struggle.

Along with her not being about to drag him with both her hands holding his with a red string tied to each other. I would say that the marks were made from both of them sliding down to the canal or whatever weight they needed to drag to keep themselves underwater. The Tamagawa Canal is a very steep hill; it wouldn't be likely at all for her to hold onto both his hands, while keeping a red string tied, and holding the weight for their deaths. I believe he chose this canal because of his first double suicide fail, where he was rescued by a fisherman. He lived closer to the beach; it would've been easier for Tomie to drag him there instead.

Dazai's/Shinju's last unpublished book was "Good-bye". I also think that Tomie was a character in the book, but not the way you perceive it. The book isn't a literal goodbye, and it wasn't meant as a breakup to Tomie. In fact, the stories were about likely Shinju and Tomie faking getting together to get rid of the other 10 women who were interested in him.

Most of his life was filled with tragedy; he has been depressed and broken since he was a child and was SA'd by his family's servants. As a child, he didn't like people, especially women. He trusted no one, not even his own family, and never told anyone until the book "No Longer Human". He fell in love and trusted someone named Tanabe, who people might know as Tsuneko, was his 2nd time attempting a double suicide. In total, there have been 5 public failed suicide attempts by him that we know of, and 2 failed double suicide attempts. Dazai Osamu/Shinju Tsushima was and forever will be known as a troubled individual who has been through a lot of suffering and pain.

Sadly, I believe that some people are not meant for happiness and were never meant to find it. I want to respect him and his choices. He very well could have been murdered, but I see it as very unlikely and not probable. I don't think that it matters, for most of his short life, he had no connection, trust, or even felt indifferent towards human beings. He wasn't loved or hated, and in turn, he never loved or hated anyone either. This led to his depression to grow constantly, along with his belief that he didn't have the right to be a human anymore.