What he's accused of doing is to kill enemy soldiers during a war. Surely it's a soldier's duty to kill enemy soldiers whenever possible? Incidentally, the US is only accusing him of being an "enemy combatant", but is treating him as though he were an "unlawful combatant".
The other question in his case is whether the Canadian government ought to request that he be turned over to Canadian courts, and many Canadians think so, many others think we ought to let the US deal with him. My view differs from both of those. When a Canadian joins an army we're at war with, then it becomes his duty to kill Canadian soldiers whenever possible, and in my view he thereby gives up Canadian citizenship. Any other view leads to such impossible contradictions, I have trouble comprehending how the prevailing view has remained so long unchallenged.