The reasoning behind Nie being true neutral goes from back to front.
It's not that she's just true neutral. Her arc before the whole ordeal made her into true neutral due to the following:
1: She's Elfkin
Elven have a long lasting life, in general they see things differently from other races, one of those is the passage of time. Time may make elves seem emotionless, but they may just be used to seeing lots of people pass by, they may not feel emotionally attached to someone unless they're fundamental to their life.
2. Nie is a diviner
Nie has had visions ever since she was a child, she's gifted in this and has protected their own for centuries. This also means she has seen hundreds of thousands live and die. Her visions are not just a way into the likely future. They're a declaration of the way of nature itself. Who is she to say who lives and dies?
3. Nie is in a state of Тоска
A deep grief of the soul as she is unable to find her sister, whether it's on real life, or on her visions. She only sees a blurry picture, which makes her have hope that perhaps Ekaterina may still be alive after 300 years.
4. Nie is a sage
After spending hundreds of years living inside a monastery, learning about religion, God and the scriptures, she has taken a different stance around life, even if she's able to see and interact with life around her. On a general note, she thinks everything that happens is God's plan. Good and bad things are bound to life.