In William Faulkner's short story, "A Rose for Emily," there are several themes.
According to eNotes.com, three themes are death, the decline of the Old South, and community vs. isolation. While all three are important, the theme I believe to be the most important is community vs. isolation.
In terms of death, we learn of the death of Miss Emily, and later, shockingly, of Homer Baron. The decline of the Old South is seen as Miss Emily turns her back on the traditions of the society in which she was raised in order to go out riding with a common worker (Homer Baron), to live alone rather than with female companionship after the death of her father, etc.
However, it is the theme of community vs. isolation that provides Miss Emily with the ability to set the terms of her daily existence and cover up a murder for so many years. Miss Emily becomes reclusive after her father's death and becomes a law unto herself. She refuses to pay taxes, stands up to those who try to collect the taxes, intimidates community members into quietly, secretly removing a terrible smell emanating from her property, and covering up her murder of Homer Baron: a fact that is not discovered until after her death.
In this classic tale of murder in the Old South, Faulkner uses these themes to weave his tale and still surprise the reader with the final sentence of his story.