The Geneva Convention rules defining "soldiers", "partisans", etc (and thereby defining those who fight outside any of the above catagories as an "illegal combatant" are rather more extensive than that). The fact that al-Qaeda thugs wear no uniform is one of their violations, but far from the only one.
Under international law, the perps of the Malmedy Massacre were entitled to no process of law beyond "yep, we've proven beyond reasonable doubt that they're SS, all right -- hang 'em high". The US went well beyond that, and McCarthy's defense of the Malmedy perps thus has no rule-of-law justification.