John Rosenthal provides background here on the evidence against John Demjanjuk, who after years of legal wrangling was finally deported from the United States to Germany to stand trial for Nazi war crimes. Rosenthal argues that Demjanjuk truly was a small fish: a captured Red Army soldier who survived by becoming a prison helper in a concentration camp. He presumably collaborated in order to avoid starving to death (an estimated half of all Soviet prisoners of war died in captivity).
Demanyuk was neither a Nazi nor a member of the SS. Rosenthal argues that the German government in this case is violating its own rule not to chase after the small fry. Previously, they have classified as small fry the shooters in the death squads that murdered millions in the east.
At the same time, the German government refuses to extradite seven former Nazis to Italy. In 2005, an Italian court convicted them of murder for participating in the slaughter of 560 civilians during the war, 80% of whom were women, children and the elderly.