Tell that to the survivors of Aushwitz...
or Treblinka. Hitler and his thugs would still be in power and still killing people had it not been for war. How about Pol Pot's unique form of insanity? His autogenocide killed an estimated 1-3 million people before an invasion by Vietnam deposed him. How about Idi Amin of Uganda who killed thousands of people until being deposed by the Tanzanian army. War ended all of the killing by those regimes, just as getting rid of Saddam will end his mass murder.