If the first one is looking you in the eyes and 10 feet from you as you pull the trigger, it’s traumatic.
I guess. I remember the first time I ever killed a deer. I was in college & in Army ROTC. The year was 1977. I had him real close. Close enough for us to look into each others eyes in tight brush. I started to shake. Classic case of Buck Fever. Then I settled down, closed my eyes and concentrated on believing I was looking at a Russian soldier. It was dead easy after that. One shot. One kill.
I've spoken to other folks who've had that moment you mention but the first one was instantly followed by numbers two through eight. The first one didn't cause any problems thereafter, except possibly for those who've been raised by liberals and democrats and all those issues. OTOH, I also knew a guy whose first kill was an insurgent he was forced to kill silently and he did so with a knife. I think that one caused him some sleepless nights but mostly because he was thinking how easily the roles might have been reversed.