You won't find the answer in a book, each individual and each situation can be a bit different. I have been disturbed, disinterested and excited depending on the mood and the context of the fight. I have seen others rejoice in it, and knew one soldier so disturbed by killing someone he eventually killed himself. I think the situation in a COIN/Stability situation is more complex than when you're involved in a battle, based on your interaction with the local community. When innocents you're trying protect are killed in a firefight with hostiles you're trying to kill, that can have a negative effect emotionally. Maybe that isn't what we're talking about here, but it is something that needs to be considered.

There is probably a sociological aspect to killing in combat that may point to norms, but ultimately it comes down to each individual's psychological make up, and how he judges each situation. I recall reading an article about a SF team Sergeant a few years back discussing the incident where he killed an insurgent in hand to hand combat and he was very concerned that his son would find out what he did. I can't speak for how he felt, but obviously he was a mature and moral individual who strived to teach and model values for his son, while in the same situation a 19 marine may rejoice and post pictures on the internet if he had them. It just depends on the person. I am happy to see some of the academic studies criticized, because they sure as heck didn't match up with my experiences.