Many Catholics oppose the use of capital punishment–at least in countries like the United States–on the grounds that it is no longer necessary to protect society from the wrongdoer. This is a reasonable argument, and it is certainly true that a Christian will prefer to see the use of bloodless means where they are sufficient.
On the other hand, proponents of this view sometimes argue as if this argument is more powerful, more open and shut, than it really is or can be. I mean that they sometimes talk as if it is just obvious that capital punishment should not only be used sparingly, but that it should be completely abolished, because it is never necessary to defend society in a country with highly effective prisons like ours.
These claims tend to overlook that it is still possible to escape from prison, even from our very secure prisons. This, in fact, just happened in the State of New York. Two men serving long sentences for already having committed murder have escaped and are on the loose. All the public authorities are issuing warnings that these men are very dangerous.
Capital punishment would serve to protect society from such men in two ways. First, obviously, if it had been imposed on them in the first place for their original crimes–which were sufficiently grave to merit it–they would be in no position to threaten innocent people now. In the second place, the possibility of capital punishment for any further crimes they might commit would act as a deterrent and protect people from them. One of them was serving a life sentence. In the absence of capital punishment, the law has punished this man to the maximum extent allowable, and there is no further punishment that can be imposed on him for any crimes he might commit while free from prison.
Here it is worth trying to clarify one possible area of confusion. In deciding to impose capital punishment on an offender, with a view to protecting society, the public authorities responsible for the decision will do so because, in their judgment, the offender in question is now and will continue to be dangerous to others. This is not the same thing as deciding that he will try to kill anyone he comes into contact with. It means judging that he is violently unpredictable and he might well try to kill someone, given the circumstances in which he killed the first time. Sometimes Catholics opposed to the death penalty suggest that imposing it amounts to deciding that the offender in question is beyond forgiveness and redemption. But it does not need to mean that at all.
If a Christian has to make a decision to impose capital punishment on a convicted criminal, he (the Christian with authority in these matters) will certainly hope that the criminal repents and seeks forgiveness before being executed. This was certainly how the matter was publicly presented, even in the old days when the public was very intolerant of crime and insisted that most murderers be condemned to death. This is why the public always made sure that the prisoner had access to clergy up to the time of execution.
In any case, when the responsible public authority decides that it is necessary, for the public safely, to execute a criminal, that authority is not deciding that this person is not capable of repentance. The authority is merely judging, based on the human evidence available, that the prisoner in question is a hardened, habitually violent offender who is dangerous to innocent people who deserve to be protected.
A person responsible for the public good could believe A) that this offender might well reform, given, say, ten more years of life, and, at the same time, B) that this offender is currently so dangerous that he might well harm or kill someone else in prison in the next ten years. Someone who believed that would not be wrong in opting to impose capital punishment, in order to protect the innocent.