Murder is, or should be, a state issue. States that don't have capital punishment obviously will opt for life without parole.
If he were squeezed, the Pope would say that in "this day and age" capital punishment should almost never be an option. But, you're right, the issue is not part of the deposit of faith and one can disagree with the Pope on this one and still be a very good Catholic.
<> Is the Catechism teaching on the Death Penalty Doctrine or not?
Can a Catholic reject Doctrine and be considered a "good Catholic?" Cite a Magisterial Document to back-up your assertion<>