Take the case where someone is suddenly surrounded in the street, forced into a vehicle and taken away. That's normally called kidnapping. But if police do it, it's called an arrest. If a person is held against their will in a cold small cell and their movements and actions strictly limited, and treated like an animal or child most of us would consider that cruel and inhumane and label it as slavery. But if it is inside a "prison" and said person had violated a law and that is our justification for them being there then it is "okay" and not evil to be doing that to them.
And almost all societies have been doing this for thousands of years. Nothing new here. Move along.
But one day the issue of waterboarding comes up, as it relates to US treatment of some terrorists caught in the aftermath of the 9/11 attacks. Is it unpleasant? You bet. Extremely? No doubt. But is the US government just showing up outside John & Jane Smith's house in Iowa, kidnapping the wife, a perfectly innocent suburban house mom, taking her away to a secret location and then waterboarding her in order to make her reveal her secret cookie recipe? Heck no. That's not what was happening. Far from it, according to the published reports. Instead, the US government used this technique only on "bad guys" -- and by that term I mean folks for whom there was enough evidence that any reasonable person would agree that he had been engaged in evil & illegal acts such is killing or planning to kill innocent US non-combatant civilians. Such as Khalid Sheikh Mohammed, the planning mastermind behind the 9/11 operation. Am I going to lose any sleep at all over him being waterboarded IF the goal was getting useful information out of him to further fight/dismantle Al Qaeda and prevent future murders? Heck no. Do I think it is wrong for him to be waterboarded? I'm not sure. At best it's in some shades of grey area. I think kidnapping is wrong, I think slavery is wrong and I think murder is wrong but every single day in this country and in many countries around the world citizens already give their governments a free pass at doing exactly these things. And we as citizens allow it (assuming democracies, people have ultimate control over laws, etc.) because even though in the general case we detest it, we generally agree there are special cases where it's acceptable because it's a choice of a lesser evil and also that some greater good is achieved because of it. This is exactly the argument behind waterboarding.
Do I like it? Of course not, I don't think any empathic, non-sadistic person would. But to call it torture and thus because it's torture it is inherently evil and therefore that there can never be any possible justification for it -- I find that an illogical and therefore invalid argument. At best, hypocritically inconsistent.
If anything I think the better case against waterboarding is whether it's effective. If it is not effective, either at extracting true and timely and useful information out of the subject, OR, in acting as a deterrent to other would-be conspirators, then that's a reasonable argument for dropping it. Another good argument is that it might drastically increase the chance that US soldiers, when captured by enemies, are themselves subjected to it. But at least with these arguments, there is nuance and there are trade-offs being considered. But to simply say X-is-always-wrong-and-should-never-be-done I find that to be a very weak argument considering all the other precedents that run in the opposite direction.