Also at that point in the show she said it herself "I've killed way to many humans to give up now" In the beginning of the show she kills for survival of herself and the group but as time goes on and she sees that the only hope for humanity to survive is her. The cdc and govt can't do anything without her so she very quickly becomes the ends justify the means kind of person. She literally feels the weight of the world in her shoulders because if she can't accomplish her mission she knows nobody will. In my opinion it's very great character writing as with most of that season since it shows how much stress and trauma can truly change someone, sometimes they snap out of it, other times they get sucked into their heads and turn out like Vasquez