Over the years I have offered many arguments for capital punishment for murder:
1. It is a cosmic injustice to allow a murderer to keep his life.
2. Killing murderers is society's only way to teach how terrible murder is. The only real way a society can express its revulsion at any criminal behavior is through the punishment it metes out. If murderers all got 10 years in prison and thieves all got 20 years in prison, that would be society's way of saying that thievery is worse than murder. A society that kills murderers is saying that murder is more heinous a crime than a society that keeps all its murderers alive.
3. It can, if widely enacted, deter some murders. Though I regard this as a less important argument than the first two, there is no doubt that it is true. Everyone acknowledges that punishments can deter all other crimes -- why wouldn't capital punishment deter some murders? Is murder the only crime unaffected by punishment?
The great thinker Ernest van den Haag brilliantly made the case for execution as deterrence: Imagine if a state announced that murders committed Mondays, Wednesdays and Fridays would be punishable by execution and murders committed the other days of the week would be punishable by imprisonment. Would murder rates remain the same as they are now on all the days of the week? I doubt it.
The most common objection opponents offer against capital punishment is that innocents may be executed.
My answer has always been that this is so rare (I do not know of a proved case of mistaken execution in America in the last 50 years) that society must be prepared to pay that terrible price. Why? Among other reasons, because more innocents will be killed by murderers who are not executed (in prison, or once released or if they escape) than will be killed by the state in erroneous executions.
So, yes, I acknowledge the possibility of an innocent being killed by the state because of a mistaken murder conviction. But we often have the tragedy of innocents dying because of a social policy. I support higher speed limits even when shown that they lead to more traffic fatalities. I support the right of people to drink alcohol even though the amount of violence directly emanating from alcohol consumption -- from drunk drivers to spousal and child abuse -- is so high.
And now I have an additional argument. Regarding murder, it is not only those of us who support capital punishment who support a policy that can lead to the killing of innocents. So do almost all those opposed to capital punishment. Nearly all opponents of capital punishment (and many supporters of capital punishment) believe that if the police obtained evidence illegally, the conviction of a murderer should be overturned.