I think Mary Anne in this excellent short story can be said to symbolise the way that you lose yourself to war. The author himself says that the war became a kind of drug to her, and she realised just how empty her life was in the United States. The war in Vietnam gave her life a meaning and a purpose that she had never experienced before.
Mary Anne's character serves two purposes. One, her character exemplifies the dehumanization of war. The more time the soldiers spent in the war, the more desensitized they became to the sights and horrors of war, such as death, murder, violence, etc. In losing this sensitivity, the soldiers also lost some of their humanity in that they began to lose the moral elements that make us human, such as sympathy, compassion, respect for life, etc. Mary Anne becomes a savage killer, enjoying the thrill of an ambush, and thus showing that she has been desensitized, demoralized, and therefore dehumanized.
Two, her character also shows the loss of innocence experienced by the young troops. The vast majority of these men were between eighteen and twenty-one years old. They were still inexperienced, young, naive boys when they entered the war, and their experiences at war changed them drastically and stripped them of that youthful innocence. Mary Anne came to the war an innocent, naive young girl who knew nothing of what she was stepping into, and in time lost that innocence.