In fact, the Church can and has changed its teaching on the death penalty, and it can and does (now) teach that it is intrinsically wrong (not merely prudentially inadvisable). Both John Paul II in Evangelium Vitae and the Catechism reject killing AS A PENALTY, i.e., as a punishment, i.e., for retributive reasons. Rightly or wrongly (I think rightly, but the teaching is not infallibly proposed—Professor Feser is right about that—nor was the teaching it replaces infallibly proposed) the Church now teaches that the only reason for which you can kill someone who has committed a heinous crime is for self-defense and the defense of innocent third parties. You can’t kill him AS A PUNISHMENT, even if he’s Hitler or Osama bin Laden, once you’ve got him effectively and permanently disabled from committing further heinous crimes. There is no other way to read Evangelium Vitae and the Catechism. The interesting debate, I think, is about the status of the eariler teaching and what kind of assent, if any, it demanded of faithful Catholics. The same is true of the earlier teaching—plainly contradicted by the teaching of (I think) all the 20th century popes who opined on the issue—that among the valid justifications for the use of military force are retributive justifications. The Church used to teach that going to war to punish aggressor nations can be justified. It now teaches that going to war to punish aggressor nations cannot be justified (though going to war to counter their aggression can be justified).