First, it is not true that you HAVE to be opposed to capital punishment. Catholic doctrine is more nuanced than that. From the official Catechism:
2266 The State’s effort to contain the spread of behaviors injurious to
human rights and the fundamental rules of civil coexistence corresponds to the requirement of watching over the common good. Legitimate public authority has the right and duty to inflict penalties commensurate with the gravity of the crime. the primary scope of the penalty is to redress the disorder caused by the offense. When his punishment is voluntarily accepted by the offender, it takes on the value of expiation. Moreover, punishment, in addition to preserving public order and the safety of persons, has a medicinal scope: as far as possible it should contribute to the correction of the offender.67
2267 The traditional teaching of the Church does not exclude, presupposing full ascertainment of the identity and responsibility of the offender, recourse to the death penalty, when this is the only practicable way to defend the lives of human beings
effectively against the aggressor.
“If, instead, bloodless means are sufficient to defend against the aggressor and to protect the safety of persons, public authority should limit itself to such means, because they better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person.
“Today, in fact, given the means at the State’s disposal to effectively repress crime by rendering
inoffensive the one who has committed it, without depriving him definitively of the possibility of redeeming himself, cases of absolute necessity for
suppression of the offender ‘today … are very rare, if not practically non-existent.'[John Paul II, Evangelium vitae 56.]
And, yes, I am going to argue that if an incarcerated individual is likely to offend again – at personal risk to prison staff AND OTHER PRISONERS – the death penalty is justified, if not required to avoid other innocent blood being shed. We haven’t the means to “effectively repress crime by rendering inoffensive the one who has committed it.” Not even close.
In prisons, there will always be low-level risk prisoners – not naturally violent, may have committed crimes while under the influence of some chemicals, may be somewhat impulsive.
There are medium-risk prisoners – those who have a long career of crime, who may have used violence or the threat of it to get their way. Those prisoners MIGHT reform and lead a crime-free life when released. This also includes that “one-time” offenders, whose decisions led to someone’s death.
The highest-risk prisoners are the ones held in super-max prisons. They have demonstrated their willingness to engage in violence without mercy. They are frequently gang-related, tatted up and down their epidermis, and – if someone was such a chucklehead as to release them – would quickly return to the usual pursuits
St. John Paul II was a nice man, a brave Cold Warrior, and a living example of personal rectitude. An expert on prisoners and recidivism, he was not. Unfortunately, some of those men do, in fact, represent a threat to society, and even to the people around them while they are waiting to die.
The death penalty is not something that we impose capriciously. Many times, a prisoner could avoid it by accepting a plea deal before trial. I would be OK with forbidding it without DNA evidence, or video evidence, or absent an eyewitness that doesn’t have a motive to lie (a prisoner testifying to get privileges or a reduction in sentence would be an example of one with a motive).