Is Emily a monster or a heroine and why?
I believe Emily falls somewhere in between these two terms. Very few people would consider her a heroine, since her anti-social behavior and snobbery were evident even before she committed murder. She was certainly a character to be pitied somewhat, in part because of the domineering upbringing by her father and the total lack of a feminine role model (her mother, for example) for her to emulate. Her monstrous aspects emerge when she is somehow scorned by her would-be beau Homer Barron. She resorts to poisoning him, secreting away the body, and then, apparently, sleeping with her one-time and forever lover. Heroine--no. Monster--maybe. Macabre--absolutely!
I believe Emily falls better into the category of an antihero. She demonstrates no discernible traits of a heroine, yet not those of a monster either.
She is a snooty, elderly woman who has loved and lost throughout her life. Her father raised her to not see men as he wanted to keep her attention at home with him. Once he passed on she falls for a man, whose sexuality is questioned. They never marry even though she feels they will. I believe when she realized he would never wed her is when she decided he would be hers for all eternity. She refuses to let go again which is demonstrated to the reader by her poisoning her lover and the preservation of his body in her home.