The Best Explanation of the Church's Teaching on Capital Punishment

Is available in this paper here. Here is a taste, which happens to be the major thrust of the argument:

Throughout the history of the Church, Catholic philosophers and theologians have said that capital punishment is licit. But they have done this without ever denying that, in a more abstract sense, any such killing goes against what is favored even by nature. Given certain conditions, capital punishment is a perfectly reasonable political expedient. Since reasonableness determines morality, this expedient found its way into human law in a relatively permanent way; indeed, there are very few Christian (or formerly Christian) nations in existence today that have not had some form of capital punishment in their legal history. But, even still, few of these nations (or their legal experts) would have resisted the argument that there is something foul or disordered about the practice. In itself killing is bad, but allowed. Mercy, in itself, is good and invoked whenever possible and appropriate.