If I thought that Catholic doctrine held the death penalty to be immoral, I would resign. I could not be part of a system that imposes it.

Lisa Miller puts him in his place:

The U.S. bishops oppose capital punishment. So do this pope, the last pope and documents from the Vatican press office. … And so, in the tradition of millions of Catholics for thousands of years, [Scalia] has rejected official teaching in favor of his own view, which he believes (to be presumptuous for a minute) to be more traditional and more moral than the established one. That’s fine with me. I don’t want a justice sitting on the Supreme Court who submits blindly to religious authority or who holds his religion above the laws of the land. So keep your job, Justice Scalia. Just don’t pretend your church approves of the death penalty. Or that you aren’t like most people of faith, cherry-picking the teachings of your church that suit you best.