When is an order a lawful order?

I got so bored, after doing things around the house today, that I sat down and watched Battle Star Galactica: Razor.
During one of the scene, the commander of the BattleStar Pegasus came across a civilian vessel that was drifting in space.  Instead of helping the crew of the drifting ship, as is the Military's charter, the Commander of the Pegasus started trying to fill the holes in her ship's crew.  At one point, the civilians didn't want to go to the Pegasus without their families, and their families didn't want to let them go.
The Commander resorted to what the British Admiralty of the 18th century would have called "Pressing" people into service.  In order to fill her crew, she ordered her soldiers to shoot the families of men who refused to volunteer for service, and take them anyway.
The scene that ensued, was one of carnage, as the civilians demanded that the Military help them, and re-iterated their refusal to join the crew.
The Commander ordered her soldiers to open fire, and when it was over, none of the civilians were left alive.

That got me to thinking.  At what point do orders from a superior officer become less than lawful orders?  What do soldiers do when they are given unlawful orders?

I'd like to hear from other people, especially soldiers, on what they think.