While the Catechism technically permits the death penalty, it, and the bishops, foresee its use as rare if ever. If capital punishment is foresworn in all cases, a criminal often lives to commit another atrocity. Is society not left helpless?

There are many complexities in discussing the death penalty, because there is some tension between the traditional doctrine regarding it, and the modern pastoral setting. 
 
Unlike abortion, capital punishment is not an intrinsic moral evil for a couple of reasons. Firstly, in certain settings, the use of the death penalty has served the common good; ensuring that dangerous criminals are no longer able to cause harm. In punishing grave offenders, others can be deterred from capital crimes too. Secondly, Scripture does not forbid the practice. Even in the New Testament, St. Paul speaks to the State’s right to punish grave offenders in this way, and even indicates that, in so doing, it acts as a minister of God's justice on the wrong‐doer. (see Rom 13:4). 
 
The Church cannot simply overrule scripture and declare intrinsically evil, what God permits in certain circumstances. However, that Scripture permits the death penalty in certain circumstances, does not mean that it is always wise or prudent to promote such punishment. 
 
In the modern pastoral setting, recent Popes, and the bishops of the world, have taught that recourse to the death penalty should be rare if ever. A significant part of this prudential judgment is rooted in concern for what Pope John Paul called the “culture of death.” 
 
The culture of death is a mindset wherein the death or nonexistence of human beings is increasingly proposed as a solution to problems. Abortion, euthanasia, and quick recourse warfare or other violent means, along with the antilife mentality of contraception, are widely promoted in our culture as a way to solve problems. The Church stands foursquare against such thinking. 
 
And even though the death penalty has received reluctant approval in the past, the current pastoral setting seems to require that the Church stand consistently against yet another way wherein death is proposed as a solution to the regrettable problem of crime. 
 
The criminal assault that many have experienced is regrettable. More needs to be done to keep serious and threatening offenders off our streets. However, given the wider pastoral setting, it is the consensus of recent Popes and the world's bishops that standing against all facets of the culture of death is an important pastoral posture to maintain, even if our tradition does permit the death penalty in very rare circumstances.