I'm not Doctor or anything but that's something a psychotic person would say. Like a serial killer.
It's funny that people think that's her discovering some hidden strength inside her. If I remember correctly, she says that in a conversation with her mentor character. He warns her about how killing can change someone so she must be careful about it, then she says that: "Actually I was surprised at how easy it was". And yes, it is exactly what a psychopath would say, so it is baffling that some people think it is actually developing her in a positive light.
She becomes a fucking master assassin in no time, and you even start feeling for the enemies. They are supposed to be a cult of crazy murderers, but some of them just have casual conversations about how she's picking them off and killing them mercilessly and they begin to doubt their own intentions while making her sound as the real fucking demon in the island. Just another instance of hilariously broken dialogue. These guys are marooned in a island and can't get out. They are painted as a crazy cult following an even crazier leader who believes in and possesses supernatural forces (which exist in the game). They are scavengers that have lost their sanity and given way to madness to perform rituals and kill in horrific ways, yet they are written like they were a paramilitary group part of a Call of Duty game, talking about their patrol routes and stuff like that, not really like people who are way gone in the head.
I don't even want to talk about the other side characters. Just fucking poorly written boring racial stereotypes or jokes. Tough mom, ethnic guy who looks tough outside but is calm inside because he is in contact with his spirituality, infatuated nerd who dies for her, old mentor, etc. It's a joke.