They call them "slippery slopes" for a reason.
You can use any number of definition methodologies to say, rightly, that these terrorists are scum of the earth. The means don't justify the ends. That's why we have a freaking Constitution that says what the laws of the land are. And the law of the land says Congress ratifies treaties. The Geneva Conventions is a treaty, and it applies to us. Unless Congress backs us out of it.
What you are essentially doing is dumbing down the treaty to suit your purposes. The precedent of the United States has been to abide by the Geneva Conventions where the treatment of combatants taken on the battlefield. is concerned. We did so with the Viet Cong, who were "no more than criminals" to the Government that was 'elected.'
Defining them as "illegal" is spurious in a Constitutionally driven society. Yes, they're dogs. Yes, they're cowards. Yes, they engage in practices that the rest of the world should condemn but doesn't. That does not justify sinking to their level (main point) but especially when sinking to their level does no real good (secondary point) and demeans the status of the United States soldier/sailor/airman/navy dude (secondary point) and undermines the soft power of the United States to condemn reprehensible behavior. We used to have traction on that moral high ground, and now we don't. Because Bush thinks every person that hates us has information on a briefcase nuke being smuggled from Pakistan into the US.
This is Torquemada stuff, man! It's heinous. It's the most heinous thing I've ever seen my country do, and it should make everyone sick, but most folks aren't paying attention because of the Orwellian language of "illegal enemy combatants" and "enhanced interrogation" and "waterboarding". It's freaking torture, done in your name and mine, and I'll have no part of it.
It's not about them.
It's about us and our standards.
It's about a Constitutional system of institutions and not cowboy autocracy.
It's about right vs. wrong. And the ends don't justify the means.
"self-aggrandizing jackass" - it says it right on the label