You don't get it. Mass murderers always have a pretext. Hitler had some justification or other, but we still hanged his generals for invading Poland. Starting an offensive war is a crime against the peace, even if you can find a piece of paper that says you were already at war.
"No one has ever argued that Saddam was co-operating with the inspectors."
The inspectors themselves argued this. You have been lied to. You should look into it and find out who lied to you and why. A lot of people died because of this.