One of the greatest documents ever written against torture, and central to understanding why the Enlightenment opposed it, is Voltaire's Treatise on Tolerance. The case in question was the 1762 arrest, torture and execution of the Languedoc peasant Jean Calas, who was accused of murdering his own son in order to punish him for converting to Catholicism. This accusation was made in spite of overwhelming evidence that the boy had simply committed suicide. Voltaire defended Calas in print and made the case a cause célèbre of the time. There is absolutely no question as to what side the founding fathers came down on, whether or not they were slave owners.
Voltaire's main focus here is, of course, against religious fanaticism. However, the important section to me is the first paragraph, which reads as follows:
We soon forget the crowd of victims who have fallen in the course of innumerable battles, not only because this is a destiny inevitable in war, but because those who thus fell might also have given death to their enemies, and did not lose their lives without defending themselves. Where the danger and the advantage are equal, our wonder ceases, and even pity itself is in some measure lessened; but where the father of an innocent family is delivered up to the hands of error, passion, or fanaticism; where the accused person has no other defense but his virtue; where the arbiters of his destiny have nothing to risk in putting him to death but their having been mistaken, and where they may murder with impunity by decree, then every one is ready to cry out, every one fears for himself, and sees that no person's life is secure in a court erected to watch over the lives of citizens, and every voice unites in demanding vengeance.
Note that what makes violence in war acceptable for Voltaire is that the participants have the same danger and advantage, and specifically can defend themselves. This is what is removed in torture. What makes torture so ethically poisonous has nothing to do with what the torture is; water boarding, ripping out finger nails, or forcing someone to stand for 18 hours all have the same effect- namely, you remove the person's ability to defend their own body from violence, and become, in effect, the arbiter of their destiny. You alienate them from human society by making them a person who can be tortured, and from themselves by removing their agency to defend themselves from abuse.
And this is why we have traditionally rejected the use of torture. And why we have to consider the ethical implications of crossing this particular Rubicon.