So, Sara would have killed herself over false information. Again, too much uncertainty about how this sword works. In a twisted way, Sara's self-hatred is what's causing the most harm, as in this chapter with Natalia's being stabbed.
"Sister I am useless without you...if I told anyone else I was fine, no one else but you would insist otherwise...won't you be here to keep me going." Exactly what I've felt about their relationship. Sara was necessary (sword or not) for Natalia's survival, and therefore the kingdom's survival. Part of the issue with shoddy utilitarian thinking is that it's difficult to measure the potential alternative outcomes: yes, things are bad now, but what would have happened to the kingdom if Sara hadn't been there and Natalia had died or been unable to muster the strength to fight? How much worse would things have been then?
That's what Natalia wants Sara to realize. Sara might have saved more of the kingdom, rather than damaged it, because she treated Natalia like a human who needed support and healing, instead of a diety. "A living child of the spirits and her guilt-ridden, sword-wielding nun" is arguably better than "a dead child of the spirits and no sword at all."