When it comes to committing a crime against humanity such as the mass murder of civilians, you're expected to refuse the order even if it means being killed for doing so. Self-preservation is not a reasonable excuse for committing a war crime, and it never has been.
OK, but try explaining that to an 18-year-old kid whose buddy / father figure / platoon sergeant has a gun to his head. Difficulty: You get to be an anonymous intellectual half a world away who might not ever find out if he makes the wrong choice. I think I can pull up 20 or 30 million data points from the last century that say it's an unrealistic expectation.
Such is the cost of being a citizen of a country that goes to war unfortunately. It's also a lesson in why everyone should be vigilant about stopping their leaders doing evil things. Some modern leaders should take note.