I'm looking for advice and input on how to write a believable psychological response to the act of killing. For context: I'm writing a post apocalyptic story where my character is trying to come to terms with the idea that she is becoming frighteningly good at killing. This particular scene is far from the first time she has killed, and she knows it will be far from the last. But before, it was always out of necessity or self defense. There have been times she killed people (but didn't feel quite so bad because they themselves were far from innocent) because she needed what they had. Afterwards, she felt unnatural, an inherent wrongness. This killing was not hard to go through with because it was vigilante justice against a really vicious murderer, but was the first time she has gone out of her way to kill when it was completely avoidable, cornered him, and looked him in the eyes without flinching. After inwardly congratulating herself on "sticking the landing" as it were, she has an existential moment where she notes with some mild, detached surprise that she feels totally okay, was even pleased just then. Then she is conflicted whether this is a welcome relief, a sign that it's going to be easier from now on, or whether she is disturbed at how simple it was. For further context, she does find it gets easier, at least for a while. For a time, she recovers from her initial trauma and copes pretty well, sometimes even with humor, to the point that the act of killing becomes almost trivial. But later, she finds herself in difficult postions where she must break eggs to accomplish her goals, sacrificing totally innocent people, doing things the old her couldn't have imagined doing, and finally finding it harder to write it off and move on. Eventually, she finds herself lying and manipulating those who are closest to her while acting as a double agent, eventually deteriorating into addiction and burning her bridges. This individual's death is not something she ever regrets, but its significance is that it represents the first step of a moral descent, and she is introspective and fully cognizant of this evolution throughout. Near the end of her journey, a former idealist not only has become despicable to herself, but becomes nihilistic because in hindsight still can't see that she had any better choice, and loses faith that the path of goodness is viable, or even exists.