What does Miss Emily want the arsenic for in "A Rose for Emily"?

Miss Emily purchases the arsenic to kill Homer Barron, a Yankee working man who has come to town to work on the paving of sidewalks in the town.  He is not someone whom Miss Emily's father would have approved of, finding such a person beneath her.  But Homer seems to court Emily, and they go riding together on Sundays.  Emily intends to marry him, and she buys a monogrammed toilet set and some men's clothing for him.  The people of the town think that she will be able to persuade him, although he is said not to be a marrying kind of man. He is seen going into Miss Emily's house at the kitchen door at twilight one evening and is never seen again. A short time later a bad smell begins to emanate from Miss Emily's house, and after a consultation with town officials, four men go to the house and sprinkle  lime about to get rid of whatever the smell is. At the end of the story, after Miss Emily dies and the townspeople go into the house, they find Homer, or what is left of Homer, dead in Miss Emily's bed, having clearly died a very long time ago, with the toilet set intact but tarnished and he in the nightshirt she had purchased for him. His position is such that he would be embracing someone lying next to him, and the people found a gray hair, clearly one of Miss Emily's, on the pillow next to him. 
What is never made clear, really, is whether Miss Emily kills Homer because he does not want to marry her and thus she can hang onto him forever like this, or she kills him because he does not want to marry her and this is her revenge as a spurned woman.  There are other possibilities as well.  Her complete denial of her father's death suggests that she cannot bear to be left at all, and so perhaps she loves Homer but kills him simply to preserve him.  There is also the possibility that Miss Emily, having been raised without a mother and apparently completely sheltered by her father, finds her wedding night so shocking that she decides to kill this man who has turned into what she perceives to be a beast.  The fact that Miss Emily seems to have slept all these years in the embrace of a dead man certainly establishes that her motives are not sane.