Is Christopher using twisted logic to justify actions he very well knows are wrong? Or is he proving that although common sense suggests these actions are wrong, they're actually logically sound?

This is a perfect example of the way Christopher has learned to interact through rote memorization of social customs. We can just imagine the lesson: "And then when someone cooks you dinner, you should say, 'Thank you for dinner.'" "Why should I say that?" "Because that is being polite."