You did not limit the discussion to the execution of American citizens. You said, "the rationale for the death penalty is a mystery to me." I am trying to find the most extreme cases to start a justification. People who murder millions of humans or violate treaties such as the one prohibiting development biological and chemical weapons that could be used to kill millions should be at the top of the list of criminals who should be put to death. If the most horrendous criminals who murder large numbers of people could be justifiably be put to death, why should not criminals who kill or mutiliate one person be put to death?