It's important to understand that Modernist literature was a reaction to Victorian literature, where there were strict rules of conduct and social expectations, a sense of order, and usually a clear idea of right and wrong. Victorian literature was often voluptuous and overwrought, using twenty words when one would do. In contrast, the Modernist period--provoked possibly by the worsening plight of the poor and unrepresented in industrialist societies, then solidified by the horrors of World War I, tended to be sparsely written and almost painfully honest about how chaotic and unstable the world seemed. This is but a glimpse of how we went from the massive tomes of Dickens to the slender novels of Hemingway. Modernist literature also stopped trying to provide happy endings, or even explanations, leaving its readers to determine meaning for themselves.
Much of these characteristics are apparent in "The Woman at the Store." The writing is sparse, revealing only important details instead of the luxurious, self-indulgent openings common to Victorian prose. When they find the store and the woman, they're surprised to find that the woman has aged and seems a bit crazy. Jo, who's obviously had an affair with her before, convinces her to be civil and let them stay the night. The narrator voices no approbation for this (most Victorian writers would); she simply lets it stand, unremarked.
The situation they find themselves in is chaotic. They are staying overnight with a woman and her child while her husband is "away shearing," and both the woman and child are clearly not quite right in the head. To top it off, they pull out a bottle of whiskey and sit around in a messy house getting drunk. The whole situation is unsettling and just...weird.
When the narrator and Hin see the forbidden picture the child has drawn, they find themselves sleepless, simply sitting all night, waiting for the sun to come up. We expect them to try to save or at least warn their friend Jo, who has spent the night with the woman, but sneak out of the house, and are about to ride away without a word when Jo comes out. He says, "I'll pick you up later," and they still don't say anything, simply riding away and leaving him to his fate--whatever it is.
The narrator doesn't not attempt to justify the murder, nor does she move her characters to say or do anything about it--or even to warn their friend that the same may happen to him. We, the readers, are left feeling slightly chaotic and disturbed at the end of the story. No questions are answered because, in Modernist works, there really aren't any answers. Stuff just happens.