To be Catholic and in favor of the death penalty in certain circumstances is not a contradiction. However, certainly 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. The onus of justification of the death penalty, for a Catholic, lies upon him, and he must shape his arguments carefully and respectfully in light of what the Church teaches. The presumption is against the death penalty, and a Catholic who wishes to argue in favor of the death penalty must carefully limit the circumstances and conditions, and not simply ignore the presumption and push back that the death penalty is good, period. To do so is to be defiant of the spiritual authority of the Church.
"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?