So all she has to do is to keep herself safe from the developing monstrosity outside. It is easy to say and think about such things but originally performing the thing is not easy at all.
The world in which there is no communication of any kind and no transportation is like a forest full of deadly beasts who are waiting for you. No one wants to get eaten up but to fight those who have turned into wild animals, one has to fight in the same ruthless way. To kill an animal you have to behave like one first.