In Lightning Returns: Final Fantasy XIII, Lightning can hunt entire monster species to extinction. They’re pests in Nova Crystallis, so destroying them benefits the population at large.
But after Lightning destroys the god Bhunivelze, she’ll create a new world where the Fal’Cie have no influence. Monsters also won’t exist to cause or suffer pain. So in this sense, wouldn’t it just be better to carry out her original mission, which is to gather souls? Nova Crystallis is running out of time, the world is ending. She needs to save as many people as possible. From this standpoint, hunting monsters looks like she’s wasting time.
Of course, she most likely has no idea she’ll be responsible for creating the new world. She’s essentially his assistant. But why does she help Bhunivelze? To save Serah, she must. She cares very little about the souls she’s gathering for themselves. She cares even less for whom she gathers them. In the Japanese version, Bhunivelze wishes to better understand humans to be a merciful God for them in the new world. In the English version, he only cares to remold humanity into soulless puppets. In either case, he is not merciful in Nova Crystallis. Lightning knows this but carries on to help god simply for her sister’s sake.
Lightning’s actions, like in Final Fantasy XIII and it’s sequel, suggest she’s selfish. She frames her every action in the service of saving Serah, at the expense of all others. All the information she acquires in her journey filters through her point of reference. While saving her sister is noble. she often tends to leave out bigger ethical considerations. Perhaps this is due to her lack of emotions?
But even so, how ethical is this? She’s the hero of Lightning Returns: Final Fantasy XIII, and Savior of Nova Crystallis. But is it right for her to only think of her feelings towards Serah? Ironically, without taking such actions, she could never confront the true identity of Lumina.