In the film version of Cormac McCarthy's novel, the "wife" played by Charlize Theron, looks intently at her husband (Viggo Mortensen) and speaks clearly: "They're going to rape me. They're going to rape him (meaning their young son). They're going to kill us. And then they're going to eat us."
Her death is not shown. She merely disappears one night. The sound of the riots outside their apartment, and the blurred outlines of explosions and fire, are a backdrop to her husband's realization she has gone. She is shown going out the door, and not looking back. It appears she is going to allow herself to become a victim of the brutality she feared; and perhaps by sacrificing herself she allows a window of opportunity for her son and husband to escape the city and travel to a safer place.
In the novel, there is a sequence where the husband has a dream of his wife and after awakening he thinks, "she died alone somewhere in the dark." The novel also makes it seem that the wife was on the road with them briefly; her last moments are related in one of the flashback sequences which occur throughout the novel. She tells her husband she wants to kill herself and refuses to stay until the boy wakes up in the morning. The husband assumes she uses obsidian to slit her own wrists, as he showed her to do, in case things became desperate. The next morning realizing his mother is not there, the boy merely says, "She's gone, isn't she?" suggesting he may have overheard his parents the night before.
The man also removes his wife's photograph from his wallet and leaves it behind on the roadside; this is symbolic of both honoring her "as she was" (not having to contemplate how she died) but also of wanting to let go of his memories of her, perhaps because they are too painful—or too easy a distraction from the constant vigilance needed to survive.