The narrator notes that Margot "shot at the buffalo" which indicates that she was aiming for the animal; not Francis.
. . . and Mrs. Macomber, in the car, had shot at the buffalo with the 6.5 Mannlicher as it seemed about to gore Macomber and had hit her husband about two inches up and a little to one side of the base of his skull.
Wilson is the character who provides further interpretation of the act and his analysis raises the question of whether Macomber's death was an accident or if Margot was actually trying to kill him. Wilson's commentary and Margot's reaction make the story more complex and this makes the ending more open to interpretation.
In this story, the narrator provides more insight via Wilson's thoughts than through Margot's or Macomber's. Some critics consider Wilson to be an extension of Hemingway, thus a character representing the author himself. Earlier in the story, it is Wilson who gives a relatively objective analysis of Margot and Francis following an encounter with the lion.
How should a woman act when she discovers her husband is a bloody coward? She's damn cruel but they're all cruel. They govern, of course, and to govern one has to be cruel sometimes. Still, I've seen enough of their damn terrorism.
Here, Wilson seems to have a bit of a negative view of women ("they're all cruel") although he agrees that to govern, they must be "cruel sometimes." So, he recognizes Margot's cruel treatment but understands it as sometimes necessary when women deal with such men. Near the end of the story, just before the last encounter with the buffalo, Wilson notes the change in Macomber:
More of a change than any loss of virginity. Fear gone like an operation. Something else grew in its place. Main thing a man had. Made him into a man. Women knew it too. No bloody fear.
Wilson is generally understood to be the guide in the story itself and as the story's interpreter. However, although he does think that Margot killed Macomber on purpose, the reader is still left wondering whether this was the case or not.