This was not a murder conviction as understood anywhere except the bizarre and twisted US military commission tribunal in Guantanamo. 

It was years later the US invented a crime that did not exist heretofore and exists nowhere else in civil criminal, military or international law .  We have laws prohibiting convictions  'ex post facto' crimes - see the Charter at S. 11(g) as does the US (clause 3 of Article I, Section 9 of the US Constitution) and it is the basis under which a number of Gitmo convictions have already been overturned.  Khadr's appeal has yet to be heard as the US is using every preocedural delay.

After the Military Commissions Act of 2006 was signed in October 2006, new charges were sworn against Khadr on February 2, 2007. He was charged with Murder in Violation of the Law of War - a wholly US invented crime that did not exist before 2006 and is not recognized anywhere else in the world.  A clear violation of the prohibition on 'ex post facto' crimes.