Society has always used punishment to discourage would-be criminals from unlawful action. Since society has the highest interest in preventing murder, it should use the strongest punishment available to deter murder, and that is the death penalty. If murderers are sentenced to death and executed, potential murderers will think twice before killing for fear of losing their own life.
For years, criminologists analyzed murder rates to see
if they fluctuated with the likelihood of convicted murderers being executed,
but the results were inconclusive. Then in 1973 Isaac Ehrlich employed
a new kind of analysis which produced results showing that for every inmate
who was executed, 7 lives were spared because others were deterred from
committing murder. Similar results have been produced by disciples of
Ehrlich in follow-up studies.
Moreover, even if some studies regarding deterrence are inconclusive, that is only because the death penalty is rarely used and takes years before an execution is actually carried out. Punishments which are swift and sure are the best deterrent. The fact that some states or countries which do not use the death penalty have lower murder rates than jurisdictions which do is not evidence of the failure of deterrence. States with high murder rates would have even higher rates if they did not use the death penalty.
Ernest van den Haag, a Professor of Jurisprudence at Fordham University who has studied the question of deterrence closely, wrote: "Even though statistical demonstrations are not conclusive, and perhaps cannot be, capital punishment is likely to deter more than other punishments because people fear death more than anything else. They fear most death deliberately inflicted by law and scheduled by the courts. Whatever people fear most is likely to deter most. Hence, the threat of the death penalty may deter some murderers who otherwise might not have been deterred. And surely the death penalty is the only penalty that could deter prisoners already serving a life sentence and tempted to kill a guard, or offenders about to be arrested and facing a life sentence. Perhaps they will not be deterred. But they would certainly not be deterred by anything else. We owe all the protection we can give to law enforcers exposed to special risks."
Finally, the death penalty certainly "deters" the murderer who is executed. Strictly speaking, this is a form of incapacitation, similar to the way a robber put in prison is prevented from robbing on the streets. Vicious murderers must be killed to prevent them from murdering again, either in prison, or in society if they should get out. Both as a deterrent and as a form of permanent incapacitation, the death penalty helps to prevent future crime.