I would like to think that, as Americans, every member of our military would understand that the oath they took to defend the Constitution was the ultimate order they are given. However, reality is that people join the military for lots of reasons. On one hand, it could be that you love your country so much, that service is the best way for you to show that love. On the other, you like the idea of wielding power over others.
There are going to be members of the military or a police force that don't have a problem with following unlawful orders. Consider the Chinese solders in Tiananmen Square, or the German soldiers in Auschwitz. These solders were convinced by their superiors that murdering non-combatants was justified. There will be Americans that can be convinced just the same.