I would argue soldiers don't have any more "instinct to kill" than any other normal person does when threatened with death, as if soldiers are radicalized, out-of-control murder machines who just instinctively kill without thinking. They're trained to have the ABILITY to overcome the natural inhibition to kill a human, the SKILL to kill efficiently, and—most important—they are trained to know when and when NOT to use that skill.

Think how similar the people in a war zone like Iraq or Afghanistan look and sound. Soldiers have to use discernment to kill terrorists by the hundreds without indiscriminately also targeting the townspeople whom the terrorists are terrorizing.