Any soldier may refuse an order that is illegal. The duty to refuse comes only into play when the order is not only illegal but also in violation of human rights.

That's an interesting distinction. I suppose, if one were working in a Sergeant Bilko unit, it might be quite a handy one to bear in mind. At any rate it's good to have it confirmed that, in the German Army, at any rate saying "I was ordered to do these things" would be no kind of defence when it came to torture and degrading treatment of prisoners. And I think that, as Teribus says,that is the legal situation in the British Army too, and many others.

My understanding is that, legally, that applies in the US Army too - but I am much less clear whether it is brought to the attention of personnel in any effective way. If so, I would think that would be a central plank in the defence strategy of those facing trial.
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"Compared to beheading captives in a brutal manner the things done to prisoners by American guards was more of a frat house prank. It appears that in some cases prisoners have been killed in the process of "the things done to prisoners by American guards".