My point was that inaction is not moral absolution.
Punishments act as a deterrent when the punishment is effective--that is, when the punishment is both the most likely thing to occur as a consequence *and* when the punishment is perceived as severe.
For example: in peaceful, low-crime suburbs where the population is not acclimated to violence, a violent crime is unlikely to end in justifiable homicide (self defense by killing), and so whatever the state hands out as sentence is the most likely consequence. Conversely, in violent ghettos plagued by gang turf wars, the most common practical consequence of violent crime is hazard: gang criminals are killed more often in gang wars than they're arrested. The difference between a fine, jail time, and execution in the first group is what actually happens as consequence; in the second group, it's just a bunch of bullshit they don't have time to worry about because they're more concerned with the immediate risk of death than some abstract idea of state execution..
Likewise, in rich towns, fines are bullshit; in poor towns, people can be crippled and destroyed by a $40 parking ticket. The same punishment is more or less effective depending on the culture. Community service works well where people are generally law-abiding and afraid of the legal process itself; imprisonment and executions--both exceedingly harmful--are necessary when dealing with people who have no appreciation for the law. Executions are appropriate where capital criminals do not fear long imprisonment.
We posit two situations from the above: the situation where execution is not a deterrent; and the situation in which it is.
In the situation where execution is not a deterrent, executions do not save lives. Executing an innocent man is a loss of innocent life, which is harmful and to be avoided. We are morally obligated to this.
In the situation where execution is a deterrent, executions save lives. The effectiveness of executions has two parameters: Ratio of criminals to innocents and ratio of innocents executed to innocents saved. A good system may execute 99 criminals and 1 innocent while deterring enough murder as to save the lives of 10 innocents for every 1 innocent executed. A poor system may execute 1 criminal per 1 innocent, or save 2 innocents per 1 executed.
In either case where execution is a deterrent, withdrawing execution means more innocent people die in violent crime. Where it is not a deterrent, the error factor is infinite: any 1 innocent executed has a share in 0 lives saved, 1/0 is infinite, and we cannot justify this. Thus, where it is a deterrent, we are morally obligated to have state executions; where it is not a deterrent, we are morally obligated to not have state executions.
This does not go away when execution is not a deterrent. Imprisonment is harmful: a man imprisoned during a critical part of his life will lose or never develop his family and career, while becoming distant with his friends and financially ruined. We thus face the same: rather than executed versus save, the numbers are imprisoned versus saved: a poor system may imprison as many innocent people as criminals!
We cannot solve this by eliminating state execution. We must instead improve our system, both in swiftness and in accuracy. Some believe we execute 1 innocent man for every 24 violent criminals; this should become 1 innocent man for every 99 violent criminals, and then even higher. We should likewise attempt to stay execution where we feel it not to be a deterrent, and carry it out where we feel it is; this will increase the overall effectiveness, saving more innocent lives per execution, of which inadvertently executed innocents have a share.
We can improve in this way by improving the stricture of evidence required for execution; but we would gain the most benefit from improving the system wholesale. Such improvement will reduce the number of innocents in prison as well as the number of innocents executed. Likewise, expedition will reduce the phenomena of innocent men sitting in jail for years at a time while facing trial, as with Casey Anthony who was sentenced to 3 years imprisonment and immediately released as she had served 4 during trial despite not having yet been found guilty of anything and with no restitution for that last year.
These are our moral obligations. We are obligate to provide for the security of society, not for our personal comfort in what our actions mean. The man who allows the destruction of innocent life to protect his sense of high morals has blood on his hands.