Given that military training involves learning to think of “the enemy” as not human (like “us”), and so should be killed, and that one must obey orders without question. Why is it so suprising that these forces face problems when they leave service?
You have an unrealistic idea of what the military is and does. I was involved with the US Army in one capacity or another for over 34 years, and they never taught anyone to think of those we were fighting as “not human.” Lawful orders are to be obeyed because failure to do so can result in catastrophic mission failure, but unlawful orders must be disobeyed or the soldier faces court martial. The reasons troops returning home face problems are many and very complex: having taken life, having seen friends killed in gruesome ways, having been separated from famiiy, having had a wife/husband or girlfriend/boyfriend desert them, having former friends turn on them, sometimes just the fact that they have experienced warfare in all its terror while “the world” back home hasn’t changed is sufficient.
