There was a fascinating documentary on this a few years back. Turns out humans really, really, really do not want to kill humans (despite anecdata) which was a problem for the military.
That reminds me of the old joke about the three candidates to join the SAS - two men and a woman. As their final challenge to prove they can unswervingly follow instructions from their superiors, however upsetting, they're given a gun and told to kill the person tied to a chair in the middle of a locked room. When they go in the room, they discover that the bound up 'victim' is their own spouse.
One man goes in, they hear a loud wail from outside as he sees his wife and he instantly realises that he could never do it. He unties her, walks out of the room with her and is told by the examiner that his application to join has been unsuccessful.
The second man goes in, an enormously loud gasp of shock can be heard from outside as he sees his wife. He sobs at the very thought of it, unties her, they walk out together and his application has failed.
The woman goes in, sees her husband and about a hundred dull clicks are heard by the examiner outside. He then hears the sound of ropes being untied and then hysterical screaming, before absolute silence. She walks out of the room and sneers "Call yourself the elites?! There's no way I'd want to join you bunch of muppets now. That gun you gave me was empty - you left me with no alternative but to beat him to death with the chair!"