I don’t think you have a grasp on anything. This story is about a deal with a devil. Tsurara let Shiho live because she did something the demon didn’t do in her life that moved her: show gratitude for seeing her love one last time. Tsurara ALWAYS had the power to make this ending happen, but she wanted to trick someone like her past self. Without that catharsis, dying wouldn’t be satisfying.
Besides, it’s not like there is some time limit. There is always another person to prey on, and that “goddess” has all the time in the world…