"a Catholic who is in favor of the death penalty must take very seriously and consider carefully what the Popes and the theological leaders of the Church have had to say"
Certainly, but what Catechism says is binding for a Catholic, and what it says about death penalty is very clear to my mind:
--- CC 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, non-lethal 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 non-existent."-----
Now, interesting question is this: are all Catholics who favour death penalty heretics? And is their salvation in peril? I don't know the answer. I think I'll have to deepen my theological knowledge. You are allowed to disagree with the Church on various questions (like war in Irag, Pope was against it, I was for it - no problem), but are you allowed to disagree with the opinions stated in Catechism?
To favor the death penalty in the limited instances in the catechism is not to be a heretic.
Example. There are convicted murderers in prison who are evil, vicious creatures. In prison they kill again. They kill and maim other guards and prisoners. Even in solitary confinement, they sometimes hurt the guards who still have to feed them and otherwise watch them. Dr. Lecter may not really exist, but there are murderous Mr. T's out there, for whom rehabilitation is beyond our knowledge, and incarceration does NOT protect other people from being killed or maimed.
That would be an example of a case in which the death penalty might well be warranted.
Another case might be vicious terrorists who murder many, but who are cool as leopards when sitting in prison for their crimes. Through various lines, they communicate with their murderous terrorist allies outside, and thereby continue to orchestrate murder. And if put into solitary confinement, they become causes celebres for their evil associates to capture, torture and murder other people to extract their release. Executing these murderers in jail prevents their being objectives in a spreading game of murder to acquire their release.
This might be a second example of a case where the murderer, even in a cage, is the cause of more bloodshed and horror, and where the doctrine of the lesser evil would permit the application of the death penalty, which is not itself illicit.
Also, I don't see how this position on the death penalty is any different, in substance, from the Catholic position on war, and specifically on the war in Iraq at the time of its inception.