Concerning Faulkner's "A Rose for Emily," a consensus exists among critics, teachers, etc., about Emily's motivation for killing Homer. Emily is obsessed with holding on to the past and avoiding change. She was raised in the ante-bellum South, was part of a prominent family living in a prominent house, and was important and central to the town and local economy. All that is past. She is a type or symbol of and for what the South endured following the Civil War. As such, she refuses to let go of anything and everything. She won't even let a mailbox be placed on her house.
In short, Emily knows Homer is not the settling-down kind of guy, and she kills him so that she can be with him forever. She refuses to let go.
In my opinion, the most logical reason for her to have killed him is that he was no longer in love with her or he was going to leave for some other reason.
In the story, Emily is kind of dominated by her dad. She can't have any kind of a normal life because of him. He chases off all her hopes for love with that whip.
But now with Homer, she has a chance, and maybe she feels that it is her only chance or her last chance. If he were going to leave her, this chance would be gone. So I think she killed him to prevent that.
Miss Emily may have been devastated that a second man in her life was leaving her. She had been a daddy's girl and when he had died she had trouble allowing his body to be removed from the house. By killing Homer, who was probably planning to leave her, she could ensure that he would remain with her as long as she chose.
The murder was premeditated because she had purposely gone to the store to purchase arsenic. She held herself above the law because she did not really feel that she needed to give a reason for wanting the poison. Miss Emily, in her own mind, probably felt that she had the right to do what she was doing. After all, she was the proud daughter of the captain.