Professor Bainbridge has a thoughtful post on Justices' religious faith and their legal decisions.
I can't endorse Prof. Bainbridge's questions for Senators to ask Judge Roberts. They are incomplete, focusing on whether "formal cooperation with evil" would require recusal, but without giving or asking for a definition of the terms; asking how Judge Roberts would decide what constitutes absolute or intrinsic "evil", nor asking what Roberts thinks his obligations would be as a Catholic if recusal were not required in a case involving such evil.
I believe a "serious Catholic" would feel obligated to actively oppose laws and decisions that his Church declares to be "intrinsically unjust." Sitting on the Supreme court would increase the duty. That would mean participating in the case and voting in a manner that would support eliminating or greatly limiting such evil. So, I ask again,"is John Roberts a Serious Catholic," and what are the ramifications if he is?
You can be a good Catholic and think that the death penalty should be done away with entirely, and you can be a good Catholic and think that it should be applied more often than "rarely."
You are not bound in conscience to adopt one position over the other. You are free to make your own prudential determination--but you are not free to say that someone whose prudential determination differs from yours is therefore a "bad Catholic."
The Church does not mandate opposition to the death penalty, nor does she mandate support for it. This means that capital punishment cannot be listed as a "non-negotiable" moral issue, and that is why it is not mentioned in our "Voter's Guide for Serious Catholics."