(I've made a point of this in a few places on the site now, but just you know here too, my internet has gone down due to some undefined screw up on the part of my ISP, meaning that I had to blow money on a mobile dongle just to be able to communicate with people about it and try to solve the issue. This also means that, while I'm posting here right now, I'll run out of data eventually and I'll have no real idea when it'll happen, and from that point on you guys won't hear from me at all until the issues with my ISP are solved.)

[@NuttsnBolts] pointed out some more useful thoughts about how the setting itself will cause an impact on it. This doesn't just apply to fictional circumstances either, but real situations too. A person who lives in a world where death is a constant thing around them all the time is most likely going to be desensitized to it all, which will have an effect on how they take killing someone. In those kinds of situations, they might still be hit hard by it, or they might just not feel much of anything because it's too "ordinary" for the life they have come to know all around them. Some people are immune the effects of killing for other reasons as well, such as the mentality a contract killer or a soldier will develop if it doesn't all build up and break them inside. These sort of examples don't really apply to the topic point of killing for the first time though.

Given how I know what severe depression and anxiety is like (I won't explain), I also agree that some of the symptoms they mentioned there are possible effects if they don't handle killing someone well.

The points made about whether or not it was on purpose or not also definitely have a contributing factor, yeah, and another thing which can play a part in how the person handles it, other than their own personal mental state and psychological conditions, is if they did it to defend themselves or if it was a premeditated kill. Like I said before, doing it to defend themselves can sometimes lead to the "kill or be killed" train of thought that causes them to brush what they did off, though this might not allow them to brush it off if they realize that even in a really bad scenario they still had free will and could have looked for a way out of the situation without actually killing the enemy. A person whose actively gone in for the kill and murdered someone won't be able to just brush it off like this, though they still might pass it off as "survival of the fittest"; in fact some people/characters actually justify being murderers this way.