Pope Francis clearly has identified mercy as the spiritual cornerstone of his pontificate, and the impression of deep compassion he's given over the last eight months goes a long way toward explaining his appeal. As a pastoral matter, however, it's not always easy to determine what "mercy" implies in concrete cases. In Rome right now, debate over whether a Catholic funeral ought to be held for Erich Priebke, a former Nazi SS officer responsible for the massacre of 335 Italians in 1944, including 57 Jews, illustrates the point. Priebke died Friday at the age of 100, having lived the last...