Let's review these factors a little more closely.

The first on the list is the Demands of Authority.
Grossman builds his arguments on dr. Stanley Milgram's studies on obedience and aggression. Here's the wiki article for details:
https://en.wikipedia.org/wiki/Milgram_experiment
Even our favourit Austro-Jewish psychologist used to say:
"never underestimate the power of the need to obey,"
and he was actually right in this particular case.
In these experiments some egghead in a labcoat was the demanding authority and he could ride the 65% of the subjects to administer lethal amount of electricity.
Now let's imagine this authority as someone who in the eye of the public opinion is there to order his subordinates and to be obeyed by his subordinates. Someone who got it's authority from the state through the chain of command. Someone who might even be a decorated war veteran, a soldier with high prestige. Or even someone who has the air of command, an air of authority by his nature, a born leader, inspiring and respected.
As Ardant du Picq put it in words:
"The mass needs, and we give it, leaders who have the firmness and decision of command proceeding from habit and an entire faith in their unquestionable right to command as established by tradition, law and society."
And he commands people who are trained to be obedient. Who are expected to be obedient else they'll have to face the court-martial or in certain situation sure death.
I'll add something about the training. All of my relatives, friends and acquaintances who served as conscripts told me when they spoke about their times in the army that they constantly had to perform nonsense unnecessary tasks or simple tasks made difficult by certain requirements and they all thought these were useless. But they are wrong. This is part of the training to be obedient. To perform orders without question or second-guessing even those which seem irrational to the soldier. We have a saying in the Hungarian Army: "if it's round we carry it, if it's angular we roll it, this is the Hungarian Defense Force".