The plot of Faulkner's "A Rose for Emily" begins in medias res, which is Latin for "in the middle of things." In other words, the plot begins as near to the end of the story as possible, without giving the ending away.
The story, then--just exactly what the reader needs to know to make sense of it, or just exactly what the narrator wants the reader to know--is told in flashbacks. This gives the narrator freedom to arrange the details in whatever order he wants to, since he doesn't have to tell the story in chronological order. This is what the narrator does in "A Rose for Emily."
If you were to make a list of what happens in the work in chronological order, you would see that Emily's poisoning of Homer is quite obvious. But when the order of the details is rearranged, as the narrator does, the surprise ending is foreshadowed, but not given away.