A little civics lesson folks: 

John Locke (famous political thinker) said that people enter into a contract with the government. The deal is that the government is supposed to protect them and in exchange the citizen should be loyal to that government. 

So when Khadr went and joined Al-Qaeda with his family and starting making bombs and fighting (and killing) US soldiers (our ally in the Afghanistan war) he essentially broke that contract. So the Canadian government really had no reason to protect him from the Americans using enhanced interrogation techniques on him. 

Trudeau however thinks that the government should always uphold that contract no matter what the citizen does to break it. That's why he offered the secret multi-million payout to Khadr.