When she appeared in a German court last month, Irmgard Furchner sat in a wheelchair clutching her handbag. Her mask and kerchief made it difficult to see the face of the 97-year-old — who has been charged with more than 11,000 counts of “aiding and abetting” murder during the Holocaust. The nursing-home resident was a secretary in the commandant’s office of the Stutthof concentration camp near Danzig, and is now among the latest former Nazis to be prosecuted in Germany. The defendants are in their late 90s or even centenarians, but German prosecutors and a Nazi hunter are determined that...