Cardinal Avery Dulles writes in
firstthings.com/article/2001/04/catholicism-amp-capital-punishment
"The Catholic magisterium in recent years has become increasingly vocal in opposing the practice of capital punishment. Pope John Paul II in Evangelium Vitae declared that “as a result of steady improvements in the organization of the penal system,” cases in which the execution of the offender would be absolutely necessary “are very rare, if not practically nonexistent.” Again at St. Louis in January 1999 the Pope appealed for a consensus to end the death penalty on the ground that it was “both cruel and unnecessary.” The bishops of many countries have spoken to the same effect. The United States bishops, for their part, had already declared in their majority statement of 1980 that ‘in the conditions of contemporary American society, the legitimate purposes of punishment do not justify the imposition of the death penalty.’ "
“In coming to this **prudential **conclusion, the magisterium is not changing the doctrine of the Church. The doctrine remains what it has been: that the State, in principle, has the right to impose the death penalty on persons convicted of very serious crimes. But the classical tradition held that the State
should not exercise this right when the evil effects outweigh the good effects. Thus the principle still leaves open the question whether and when the death penalty ought to be applied. The Pope and the bishops, using their
prudential judgment, have concluded that in contemporary society, at least in countries like our own, the death penalty ought not to be invoked, because, on balance,
it does more harm than good. I personally support this position.”
It is the broadness of the net consequences that matters of course, not just the specific question of whether
the death penalty was needed to protect society from this criminal or not. But in any case, comparing a good(s) of one kind with an evil(s) of another is inherently challenging.
I believe some may argue that the imposition of CP in circumstances when imprisonment is
sufficient to neutralise the threat from this criminal is not in fact CP, but
must be an act (within the class of State Executions) with an evil moral object. I believe that is wrong (noting CP/punishment is not equivalent to self-defence). Further, the “good effects versus bad effects” assessment - while including the elements of “protection from this criminal” and “death of this criminal” - is
broader than that, and their weighing is a matter of prudential judgement.
From the CDF “Donum Veritatis…”:
"When it comes to the question of interventions in the prudential order, it could happen that some Magisterial documents might not be free from all deficiencies. Bishops and their advisors have not always taken into immediate consideration every aspect or the entire complexity of a question.
But it would be contrary to the truth, if, proceeding from some particular cases, one were to conclude that the Church’s Magisterium can be habitually mistaken in its prudential judgments, or that it does not enjoy divine assistance in the integral exercise of its mission."
vatican.va/roman_curia/congregations/cfaith/documents/rc_con_cfaith_doc_19900524_theologian-vocation_en.html