I'd say give them every benefit of the doubt, but if there is strong evidence of illegal action then they have to be tried. Note also that this is almost never combat troops, but is almost always rear echelon troops. I don't have a really big problem with what was proved to have been done at Abu Graib, which roughly equates to a fraternity hazing, but you can't allow troops to break such rules, or generals to condone it.
These people by and large are combatants not covered by the Geneva Convention or by commonly accepted rules of war. But that doesn't mean you can torture them any more than you can torture other people not so covered, such as civilians. You can legally execute them, depending on how they were taken and what they have done, but you can't legally torture them. Whether or not you consider water-boarding to be torture is up to you, as opinions can vary; we do it to our own troops, but we do it to train them to resist torture. As to the other stuff alleged, remember that part of jihadi handbooks is, if captured, to claim you were tortured. When someone claims to have been beaten for months and there is no visible damage, I tend to think he's lying.
And I have no objection to hooking up battery cables to a terrorist's testicles if you have reason to believe there's an imminent threat that can be averted, but there must afterward be a court martial and you'd better be able to prove there was a damn good reason. Torture on the grounds that they have general information or can identify other terrorists should not be allowed. I am of course in favor of standard interrogation techniques such as sleep deprivation, loud music, too hot/too cold rooms, and piping in non-stop episodes of "The View".
On second thought, that last one is too inhumane even for terrorists.
EDIT: To Intellectual.Osmosis, thank you for your service. All too often people say essentially "I'm not condemning you, just everything you do and everything you stand for." In my opinion that's just a politically correct way of saying "I'm condemning you."