Here's all you need to know about Omar Khadr - you don't know and probably won't know for years at the very least. You don't know what he did in the Afghan cave and you don't know what was done to him at the time and in the years at Guantanamo that followed.
There are a few things you do know. He was a "child soldier" when he was shot twice in the back and captured. You know that even the United States is a signatory to the protocol that requires captured child soldiers to be treated as victims, not perpetrators. You know that, from the outset, the proceedings against Khadr in Guantanamo have made a farce of any notion of American justice.
So Khadr finally pleaded guilty to all charges brought against him. I suppose if the deck is stacked high enough you really don't have much choice and stacked it truly was.
Did he commit the crimes for which he was charged? I really don't know and neither do you. Is his guilty plea an admission of guilt? In these circumstances, not at all. It's an administrative not a legal act. When your captor claims the power to hold you forever regardless of charge much less guilt, there's nothing remotely legal in the decisions taken by a captive.