I don't know what set of facts you were looking at. Nothing indisputably establishes that he "[took] up arms." The only person who had alleged to have seen Khadr carrying a grenade was Mr. Layne Morris; he later recanted that.

The signed confession was coerced. Various judges at all levels of courts ruled so in 2010; and coercing confessions undermines basic Charter protections, and vitiates voluntariness/free choice. 

You have: 1. a coerced confession, and 2. an unreliable witness who recanted his statement. How are you so sure of Khadr "[taking] up arms?"