I heard it makes a big deal the first time you kill someone and then you proceed to headshot like 8 people immediately afterwards.
Sort of. After your first kill you have to kill 4 people right next to one another to get out, at least it's 4 on Normal mode. If you stealth kill the first person then you do proceed to headshot all of the people but if they see you first then you get a scene with her pleading with them to walk away that nobody has to kill anybody and whatnot. So depends on what you do and whether you get that dialogue and she cries afterwards at the camp and seems distraught in her journal.
***Slight Spoilers*** as to her attitude during the game:
I felt the pacing was good til right about the middle and then even in her journals which 1-6 are her coming to grips with the fact that she has to kill to survive or what the people are doing on the island is sick and crazy this then changes to a more cold hearted approach. It shifts to her cursing out people as she kills them in game and not flinching or even mentioning the dead bodies in this cave where they send slaves. I felt the first half there was good development to her character then they tried to speed it up to get where they wanted to go by the end of the game.