The Taliban had nothing to do with 9/11. The US and their allies (including Canada) went to Afghanistan and attacked the Taliban. We brought war to them. The US classified the Taliban as "illegal combatants" and subjected captured Taliban to torture and other inhumane treatment. No such classification is recognized by International Law.
According to international law, Either captured Taliban are POWs and subject to Geneva Conventions or they are criminals and entitled to due process.
Whenever a Canadian citizen is held in a foreign prison and subjected to torture and other inhumane treatment, the Canadian government has an obligation to demand that Canadians are treated fairly and humanely as well as intervene on their behalf. Khadr was tortured in a foreign prison and the Canadian government was complicit in that torture and ill treatment.
What Kadr is alleged to have done is besides the point. If he's a criminal he's entitled to a trial. If he is a POW he is entitled to treatment as per Geneva conventions. If he is a child soldier, he is entitled to be treated as such.
At 15, Khadr met the definition of a child soldier. Here is the way child soldiers are viewed and treated according to international laws, treaties and conventions:
Child Soldiers | Children of Conflict | BBC World Service
Is Canada a nation of laws? Khadr's case indicates that Canada just is a mob of angry people intent on lynching children.