Who killed Takehiro in "In a Grove"?

"In a Grove" is perhaps the most famous of Akutagawa Ryunosuke's works. It is the basis for Kurosawa Akira's movie Rashomon and helped cement the widely used plot line of telling varying accounts of solving a crime, a plot that is seen on both television and in film. However, using this method of storytelling also means that it can be difficult to know which details are true.
It is obvious to blame Tajomaru for the murder of Takehiro, as he is the criminal in this story. He admits to the crime. However, there are several inconsistencies within his confession that do not seem to reflect Takehito truthfully as a person. As is sworn by Masago's mother, Takehito's mother-in-law, he was a person with a gentle nature, contrary to the greedy person Tajomaru describes. This is where the reader must begin to draft their own version of events, as it seems the testifying characters are lying.
Afterwards, Masago herself takes responsibility for the crime. Her description fits the wound; a single sword had killed him. She claims to have killed Takehiro as his final request.
Next, the reader has even more to think about as the account of Takehiro himself is recorded. Through a medium, he testifies and claims to have committed suicide.
With these three accounts, the question is not so much who the murderer is but rather why the murder was committed. Each person has their reasons for claiming responsibility, so it is the reader's job then to figure out who perhaps has the strongest reason for wanting Takehiro dead. The appeal of the story is that no one can definitively say what is reality.
For Tajomaru, he can build his reputation if he is charged with the murder. He will appear stronger, a worthy criminal to his underworld friends. For Masago, she will appear more sympathetic. After all, in that era of Japan, it was seen as righteous to kill oneself after such a disgrace. By killing her husband on his own orders and then attempting to take her own life, Masago is shown as an upstanding and moral woman of the time. When Takehiro claims to have committed suicide, it is to save face. It would have been too shameful for a samurai to have been tricked by a common criminal. Suicide is the only way he would have been able to redeem his honor after such a confrontation.
So really, it is up to the reader to decide who is telling the truth.