Isaac Ehrlich's study on the deterrent effect of capital punishment in America reveals this. However, in the last year of the study, there were 22,520 murders committed and 1 execution performed. The absence of deterrence is clearly shown. The death penalty is irrevocable. In case of a mistake, the executed prisoner cannot be given another chance.
Justice can miscarry.
In the last hundred years there have been more that 75 documented cases of wrongful conviction of criminal homicide. Undoubtedly many other cases of mistaken conviction and execution occurred and remain undocumented. The belief that execution costs less than imprisonment is false. Abolitionists believe that the offender should be required to compensate the victim's family with the offender's own income from employment or community service. There is no doubt that someone can do more alive than dead.