You seem to assume that soldiers are by default OK with killing people.
Up until the middle of the last century, that wasn't the case. Most soldiers would (often intentionally) miss their targets because it turns out most humans are really hesitant about actually taking another human's life.
In part, the higher "efficiency" of the modern military comes down to dehumanising the target. In the US it is very easy to see from an outside perspective how this has spilled over into the media and public discourse -- I vividly remember several politicians explicitly talking in televised interviews about how Snowden was a traitor and should be killed without a trial, for example, a statement which seemed to spark no significant outrage and had no political consequences.
The trick is that the others aren't just "a threat", they're not even human. Terrorists don't have families. And if they do have children, they're the horrible animals dragging those innocent children into this -- the "migrants" are forcing us to separate their kids from them, it's their fault for even imposing this situation on their kids. Oh, sorry, hostility towards refugees and immigrants is of course a completely different topic; no idea how this slipped in there.
Humans hesitate when told to kill fellow humans. But they're pretty good at murdering if you train them not to consider their victims human.