Did they make sure their victims felt no pain when they killed them? Unlikely. If you are going to have the death penalty, make it hurt. They made their victims suffer. Why should they then get drugged so they feel no pain? Think "electric current". Or stoning. Remember, preventing heinous crimes before they happen is the most humane thing a civilized society can do. Making the punishment so severe that 99.9% of people would not even consider doing the crime is the most compassionate thing you can do for your citizens as a whole.