Nitric acid burns are destructive in several ways. Not only do they break down flesh and organ tissue by a very high rate of denaturing (essentially ripping the amino acids out of their protein base), but the heat generated does its part to further break down and COOK what organs are not first denatured. Nitric acid also produces nitric oxides of various forms, which destroy lungs and air passages, in a most painful manner, essentially adding asphyxiation to the death process.
And every bit of it will be excruciatingly painful.
I know. I had once worked in a chemistry laboratory, and learned this from an accidental nitric acid spill. Nitric acid eats up just about anything of organic nature, except for some kinds of rubberized material. You do not want to inhale the fumes, either.
This is so far from any form of “humane” execution, that there has to be a psychopathic element involved.
They are sending a message to future spies. Spy on us and you will experience intense pain. Meanwhile, a former US general proclaims that we need people spying on us so our enemy will not attack. Who do you think wins that game in the end ?