Dear Claire:

I moved your question to this forum since it is really not a Church History question.

The Catechism of the Catholic Church is a summary of official Church Magisterial teaching -- it is Magisterial teaching. As such it contains all four levels of Church teaching from infallible dogma, to infallible definitive doctrine, to authoritative teaching, to disciplinary teaching.

The doctrine of the Church on Capital Punishment has not changed and will not change. A society has a responsibility to protect its citizens from harm. If the only way to protect society from a murderer is the death penalty then it is appropriate to utilize the death penalty as long as the criteria of the Church is met concerning certainly of the offender, fairness in application, and humaneness in method.

However, in terms of current societal conditions, the Pope has stated that in this day in age, at least in the first world nations, that the need of the death penalty is virtually non-existent. This is also stated in the Catechism.

If that societal condition changes whereby the death penalty is needed once again, then the Church will amend that statement, but the teaching about society's responsibility to protect its citizens even by the death penalty under certain conditions will remain.

Here is the catechism on the death penalty:

2267 Assuming that the guilty party's identity and responsibility have been fully determined, the traditional teaching of the Church does not exclude recourse to the death penalty, if this is the only possible way of effectively defending human lives against the unjust aggressor.

If, however, non-lethal means are sufficient to defend and protect people's safety from the aggressor, authority will limit itself to such means, as these are more in keeping with the concrete conditions of the common good and more in conformity to the dignity of the human person.

Today, in fact, as a consequence of the possibilities which the state has for effectively preventing crime, by rendering one who has committed an offense incapable of doing harm - without definitely taking away from him the possibility of redeeming himself - the cases in which the execution of the offender is an absolute necessity "are very rare, if not practically nonexistent."

Thus, paragraph one and two is settled doctrine. Paragraph three may theoretically change with societal conditions. 

If every nation on the earth abolished the death penalty (which almost all first world nations have; only the United States and Japan still have the death penalty) this teaching remains. The duty of society to protect its citizens still is true. The need for the death penalty to accomplish that protection is still a possibility theoretically at least. What changes are the societal conditions; not the doctrine.

While the death penalty remains a theoretical possibility under Church doctrine, it has become obsolete and unnecessary is what the Pope is saying. That is not likely to change. But, if we were to have a major catastrophe such as a global nuclear war, or a major meteor strike the earth, we may be thrown back into conditions similar to the middle ages. That could necessitate the death penalty to protect society since we would no longer have the structures to incarcerate murderers as we do now.

You ask if it is morally permissible to perpetrate the death penalty when the Catechism does not expressly forbid it? But, the Catechism, as a summary of official Magisterial teaching, does forbid the death penalty unless certain conditions are true.

The death penalty is permitted "if this is the only possible way of effectively defending human lives against the unjust aggressor".

Further...

If, however, non-lethal means are sufficient to defend and protect people's safety from the aggressor, authority will limit itself to such means, as these are more in keeping with the concrete conditions of the common good and more in conformity to the dignity of the human person.

This Magisterial statement does not say that authority "may" limit itself on the death penalty when non-lethal means are available, it says that authority WILL LIMIT ITSELF.

The current practice of the death penalty in the United States is morally evil since we do have non-lethal means that are sufficient to protect our society and those who support it are promoting a moral evil.

God Bless,
Bro. Ignatius Mary