I suspect part of it is when one is expected to be a killing machine, compassion has to be subjugated.

Michael Crichton wrote in his book about medical school that in order to work with the cadavers and not get bogged down in maudlin feelings about the mostly old people they dissected and studied, they became quite callous, tossing around organs in impromptu, in the lab, football gmaes, etc. He said that was their way of dealing with the horrors of what they were doing. To an outsider it would seem completely sickening, but he said if they hadn't treated it that way AND with humour, they could never have made it through, mentally, physically and emotionally.

I am not condoning any of it, in war or school, just offering it as some possible insight.