It is clear that if you look at the parts of this story that mention the mother, what stands out is the way that she is so deliberate and resolute about her actions. Once she has decided about the hopelessness of their situation, and that there is no future for her, she is resolute about her choice to end her life and kill herself. Nothing her husband can say to her will make her change her mind. Note what the text tells us about her:
Always so deliberate, surprised by the most outlandish advents. A creation perfectly evolved to meet its own end.
Thus, when we think of the character of the mother, it her deliberateness in what she does and chooses to do that is clearly her key character trait. In a sense, although she abandons her husband and son, we can also understand her point of view. Humanity is on the brink of destruction and the only fate that she sees awaiting her is either being eaten or being kept as a kind of sex slave and seeing her husband and son eaten or worse. No wonder she was resolute about killing herself.