False Dichotomy
When you decide to start a war all dead are projected. They are not yet dead. You have to make a rational decision based on what the outcome of going to war is versus the outcome if you don't. We didn't go to all out war over the last 12 years and the people that Saddam killed are real while those war would have killed are not. Using your basis of real deaths then we should obviously have gone to war regardless of whether or not real war would have caused more deaths.
Sorry the only rational basis is to make your best estimate ahead of the decision.
Regards,
Mark