One of the problems inherent in Faulkner's "A Rose for Emily" is to identify the guiding principle of the story--the moral, if you will. Complicating this problem is Miss Emily's act of murder and then necrophilia, acts that are widely and justifiably condemned by the story's readers, and many readers see the story as a Southern Gothic horror story and not much else.
I suggest that the moral of the story is that repression of a person, in this case, Miss Emily--in any form and from whatever source--can lead to the kind of horrific consequences we discover at the end of the story. And this repression begins with Emily's father, who is depicted in a portrait as
a spraddled silhouette in the foreground, his back to her [Emily] and clutching a horsewhip, the two of them framed by the back-flung front door.
The dynamics of this portrait are startling: Emily's father in front, with his legs spraddled (indicating confidence and ease) and carrying a horsewhip, a symbol both of power and cruelty, with Emily behind him, clearly not as important as her father and in a subservient position.
We learn in the sentence before the description of the portrait that "None of the young men were quite good enough for Miss Emily and such," a clear indication that Emiily's father, not Emily, has found no one suitable for her. One of the inferences we are forced to make is that he did not want to find a suitable suitor because he wanted his daughter to remain his "property," caring for him and his house rather than allowing her to follow the normal path of a young, aristocratic southern woman--find a suitable husband, get married, have a family of her own.
Fortunately, we have a 1959 interview with Faulkner in which he is asked why he titled the story as he did. He said that he thought a woman who had "no life at all" deserved a rose and that
there was a young girl . . . who was brow-beaten and kept down by her father, a selfish man who didn't want her to leave home. . . .
Faulkner goes on to say that this repression led to the bizarre consequences he describes in the story--Miss Emily's attempt to have the life she was denied by creating a "family" of sorts through murder. Faulkner acknowledges that Emily's actions are morally indefensible but, more important, they are the consequences of the unnatural repression of her father, which led to Emily's blighted life.
In essence, then, the moral of this story is that repression, which can come in many forms, can have unintended (read, horrific) consequences. In other words, if someone's normal aspirations are crushed, those aspirations may be realized later in a terrible and destructive way.