Taking a human life, no matter whose it is, is something that we, as civilized people, are not brought up to easily accept.
Sure, there are some who, for whatever reason, seem to have that inbred ability to separate their emotions from their duties, but for the average Joe, a lot of introspection is needed.
In some services it's easier than others. For example, it is far easier to reply to "Take track 3452 with Bulldogs" and wipe out a patrol boat with 75 people on it than it is to crawl through the mud, sneak up on a single sentry, and slit his throat. In the Navy, I never saw any special training or counseling to help us deal with the former, and I regret I am unaware of any such resources being available to help people prepare for the latter.
Those in uniform (past and present) love to boast that they'd take it to the enemy no matter what, but ask any of them when no one else is listening, and you may get a very different answer.
Also, there is the difference between what you think beforehand and what actually happens when the SHTF. If you came home and saw some guy holding a knife to the throat of your daughter, would you draw and fire immediately, or would you hesitate? Would you crap your pants and beg him to kill you instead?
I submit that no one knows the answer to that question until faced with it or something similar, unless they have undergone some kind of conditioning I am unaware of, but would love to at least hear about, if not receive.
BTW, this doesn't make our troops murderers or robots. It makes them PEOPLE who are doing a mighty DAMNED hard job, and for reasons that DON'T include lots of money.