In my book on the subject, Capital Punishment & Catholic Social Teaching: A Tradition of Support, published in 2009, I wrote:

“Capital punishment as a way of protecting the innocent is one of the central issues in the social teaching of the Church, but the ambiguity about it—particularly in the United States—over the past several decades after two millennia of certainty, places the credibility of the teaching itself at risk, and that negatively impacts the Church’s social teaching as an effective tool for criminal transformation, further risking the immortal souls of those who are lost and whose being found largely relies on the constancy of the teaching of the Catholic Church, on eternally walking the eternal talking.” (p. 11)

This article from First Things also notes the ambiguity of recent statements by the Holy Father about capital punishment and Catholic teaching.

An excerpt.

Pope Francis recently gave a speech to the International Association of Penal Law advocating for the improvement of prison conditions and reiterating pleas made by his predecessors John Paul II and Benedict XVI for an end to the death penalty.

Francis, however, went further than either of his predecessors by extending Catholic critiques of capital punishment to life sentences, which he condemned as the “death penalty in disguise.” His comments have reopened debates in Italy about life sentences (nearby countries such as Spain and Portugal have abolished them) and prompted Catholic bishops in the Philippines to denounce life sentencing as “inhuman.”

Those of us who lean conservatively where criminal justice is concerned would do well to take to heart the Pope’s critique of the “vengeful trend which permeates society” and reflect on how our attitudes toward convicts line up with the teaching of Scripture. “Remember those who are in prison, as though you were in prison with them,” the author of the Letter to the Hebrews exhorts us (Heb 13:3).

The Pope’s comments should provoke a searching examination of conscience in the United States, which incarcerates a larger proportion of its population than any other country in the world. A country in which 80,000 people languish (many unnecessarily) in the psychological hell of solitary confinement should listen to a Pope who brands as “cruel, inhuman, and degrading” the practice of placing prisoners in a situation where they “lack contact with other human beings.”

That said, it is difficult to see how opposition to life imprisonment in principle could be a legitimate development of Catholic teaching. The Catholic Church has long upheld the right of a society to remove serious offenders from the community. If an offense is serious enough to warrant permanent removal from the community, this can be done either through the deprivation of life (capital punishment) or liberty (life without parole). In a 1952 allocution, Pope Pius XII taught:

In the case of the death penalty the State does not dispose of the individual’s right to life. Rather public authority limits itself to depriving the offender of the good of life in expiation for his guilt, after he, through his crime, deprived himself of his own right to life.

Retrieved November 5, 2014 from http://www.firstthings.com/web-exclusives/2014/11/the-pope-and-the-problem-of-punishment