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."
The revision states:
2267. Recourse to the death penalty on the part of legitimate authority, following a fair trial, was long considered an appropriate response to the gravity of certain crimes and an acceptable, albeit extreme, means of safeguarding the common good.
Today, however, there is an increasing awareness that the dignity of the person is not lost even after the commission of very serious crimes. In addition, a new understanding has emerged of the significance of penal sanctions imposed by the state. Lastly, more effective systems of detention have been developed, which ensure the due protection of citizens but, at the same time, do not definitively deprive the guilty of the possibility of redemption.
Consequently, the Church teaches, in the light of the Gospel, that “the death penalty is inadmissible because it is an attack on the inviolability and dignity of the person”, and she works with determination for its abolition worldwide.
The revision is quite frankly awfully written for catechetical purposes; for instance, what "new understanding" has emerged about the "significance of penal sanctions"? It's certainly not given here. The purpose of a catechism is to facilitate teaching, and this sort of useless vagueness is worthless for that. There is a letter, however, that gives some further explanation:
7. The new revision of number 2267 of the Catechism of the Catholic Church, approved by Pope Francis, situates itself in continuity with the preceding Magisterium while bringing forth a coherent development of Catholic doctrine. The new text, following the footsteps of the teaching of John Paul II in Evangelium vitæ, affirms that ending the life of a criminal as punishment for a crime is inadmissible because it attacks the dignity of the person, a dignity that is not lost even after having committed the most serious crimes. This conclusion is reached taking into account the new understanding of penal sanctions applied by the modern State, which should be oriented above all to the rehabilitation and social reintegration of the criminal. Finally, given that modern society possesses more efficient detention systems, the death penalty becomes unnecessary as protection for the life of innocent people. Certainly, it remains the duty of public authorities to defend the life of citizens, as has always been taught by the Magisterium and is confirmed by the Catechism of the Catholic Church in numbers 2265 and 2266.
8. All of this shows that the new formulation of number 2267 of the Catechism expresses an authentic development of doctrine that is not in contradiction with the prior teachings of the Magisterium. These teachings, in fact, can be explained in the light of the primary responsibility of the public authority to protect the common good in a social context in which the penal sanctions were understood differently, and had developed in an environment in which it was more difficult to guarantee that the criminal could not repeat his crime.
(Why didn't they just put the "rehabilitation and social integration" part into the revision itself?)
This shows, I think, a problem with a great deal of recent episcopal teaching on morals, that it fails to make clear distinctions between claims based on strict requirement, claims based on moral safety, claims based on general expectation, and claims based on evangelical ideals. When, for instance, it is said that something is inadmissible, does that mean that it is wrong simply speaking, that it is morally unsafe (strictly speaking permissible, but involving too much risk of violation of moral principle), that it is only rarely permissible and under circumstances that very few people will have to deal with, or that while permissible it is something we as Christians should strive to rise above and (possibly) strive to help others rise above, as well? It matters, and bishops are horribly bad at clarifying these things -- one unfortunately suspects because they want the ambiguity so that they can advance or retreat as is convenient for them. We see this in very problematic form here:
(1) The claim, "it is an attack on the inviolability and dignity of the person", would normally be read by most people as directly implying that it is simply morally wrong, by universal moral obligation.
(2) The claim that the Church teaches the inadmissibility of the death penalty "in the light of the Gospel" would normally be read by most people as implying that it is something depending on specifically Christian teaching.
(3) John Paul II's argument in Evangelium vitae is most straightforwardly read as saying that the death penalty is permissible, but only under conditions that are rare and have become so rare in the modern world that the death penalty is certainly morally unsafe. Not only does the letter reaffirm Evangelium vitae, it gives this argument explicitly.
If the letter is taken at face value, the revision is a Red Queen's Race: the CDF is running very hard to stay in the same place. It does make the section in the CCC, itself a clunky and not entirely clear summary of Evangelium vitae, more explicitly against the death penalty, but it is justified on the same basis that the original section was, and doesn't actually provide a better summary of that argument. If anything, it's more likely to cause confusion as people puzzle over the completely unhelpful explanation in the section itself.