From a psychological point of view in the animal world its rare for animals to kill their own kind, well, mammals at least.

So its take a huge amount of mental conditioning (read brainwashing) for humans to feel comfortable doing it, whether for country or for moral reasons.

These type of problems are to be expected in armed forces...

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"The first stage of the great adventure has concluded happily, and here I am installed in Mexico, although I have no idea about the future" Che Guevara