I see several issues here: First, the Catechism. Second, prudential judgement. Third, the Death Penalty, and my own opinions pertaining thereto, because, hey, this is my blog.
So first, the Catechism:
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, nonlethal 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 are 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
Fairly straightforward. The only acceptable reason for the death penalty is if there is no other way to protect people. "Guy has it coming" is not an acceptable reason.
Second: Prudential Judgement. I admit I am not entirely certain what this phrase means, although I have seen it used many, many times. Thus, even though I am not entirely certain what a prudential judgement means, I can tell how it is used. It is used almost exclusively by those who consider themselves to be politically right wing, and they use it in almost the exact same way that left wingers use the phrase "primacy of conscience": as a way to blow off a teaching of the Church with which they disagree, or perhaps find inconvenient.
Both those who identify themselves as left and those who identify as right wingers should understand that the Church is neither left nor right. Further, they should stop trying to force the Catechism to say what they want it to say and instead see what it does say, and, above all, stop teaching others how to get around the teachings they do not like. The Church's teaching here is difficult, and it calls upon the faithful to be wise and to weigh matters as best they might before deciding. Difficult teachings are the ones we are most inclined to blow off or at least sidestep. They are also the ones we should heed the most.
Finally, me. I have heard of the stories of many heinous murders, and know that monsters walk among us. I know the names of many of these monsters and their victims, and there are times when I wish upon them the most gruesome forms of vengeance my mangled soul can devise that the darkness they visited upon the innocent of this world may be doubly redoubled upon them.
But I also know other stories and other names. I know Donald Marshall, Guy Paul Morin, David Milgard, and yes, Steven Truscott. All innocent men who were found guilty beyond a reasonable doubt by their peers, and sentence to long terms in prison, and all innocent of the charges levied against them. When I think of their stories, I am profoundly grateful that Canada has abolished the death penalty.
Crimes are committed by people, then investigated by people, and judged by people. Mistakes are sometimes made. Occasionally, we send the wrong person to jail. If we had the death penalty, it is inevitably that we would from time to time kill an innocent. I find that unacceptable. It is a terrible thing to lock up an innocent man, but at least there is time to review the case, realise the mistake, and restore the man to society as best we can. There is no restoration for the dead, and an apology to a corpse is nothing but a most bitter irony.
At its heart, the Church's teaching on the death penalty is minimalist. It may be used only in extreme situations, and only if there is no other way. Those who, like the good father, endorse the death penalty, tend to be maximalist: kill early, kill fast, kill often. This they call prudential judgement. I can not believe it is so. There is no prudential judgement for slaughter.
I also believe priests should not be teaching others ways to ignore the Church's teaching.