This famous story provides us with multiple perspectives on the death of Takehiro, perspectives which contradict one another. There are three possibilities: Takehiro was killed by Tajamuro, the notorious robber, killed by his wife, Masago, or committed suicide.
Tajamuro testifies that he meant to steal Masago from Takehiro, but killed him because Masago insisted that they duel to the death, with her as the prize. Masago testifies that she kills Takehiro because since he has witnessed her dishonor, he cannot be left alive. Through a medium, Takehiro testifies that he has committed suicide. The limited physical evidence does not establish one explanation definitively, so we are left, like the police commissioner, with only the credibility and motivation of the witnesses to come to a conclusion.
Tajamuro has always struck me as a bit of a braggart, with a reputation as a "bad guy" to maintain. He clearly relishes his story, heaping detail upon detail, to show what a great fighter and lover he is. He is characterized as "cheerful" at one point as he tells his story, and later he is described as "defiant." (I apologize for no page number citations, but I cannot find my copy of the story. Fortunately, I know the story well enough to know these are accurate quotations.)
Masago is very young, and her mother describes her as a "spirited, fun-loving girl." She also takes the time to say that she is sure Masago has never been with a man other than her husband. These details from the mother have always made me wonder how spirited Masago is and why her mother needs to say anything at all about Masago not having been with any other man. Surely this was presumed in that time and place? I think Masago might be a bit of a flirt and a troublemaker, and she could have concocted her story to show herself in the most innocent light after a dalliance with Tajamuro.
Assuming one suspends one's disbelief in mediums, Takehiro's testimony is probably the least self-serving. He relates that Masago agrees to go willingly with Tajamuro, asking first that he kill Takehiro. Tajamuro declines to do so, offering to kill Masago for Takehiro, since she has betrayed her husband so callously. Takehiro forgives Tajamuro because of this. It seems that Takehiro kills himself in shame, anger, and jealousy. A case can be made for any of the three possibilities, but it is my opinion that Takehiro commits suicide because his testimony is the least self-serving of the three.
The author had no intention of providing a definitive resolution to this death, but rather makes the point that our memories are frail and colored by emotion and motivation, with no way of defining "truth."