Klaus Braun is the ultimate antagonist of the CSI: NY Season 5 episode Yahrzeit. He posed in America as a gentle Jewish Holocaust survivor called Abraham Klein, but beneath his warm and gentle disguise, he was in actuality an evil former Nazi responsible for betraying a family of Jewish refugees to Auschwitz during the Final Solution.
Klaus Braun lived in Strausberg during the Nazi regime in Germany. Klaus' father was an SS engineer at Auschwitz who built the camp's crematoria, and Klaus himself was a member of the Hitler Youth from 1941 to 1943.
During the Final Solution in which the Nazis began rounding up and exterminating Jews, Klaus' Jewish neighbours, Esther Schnitzler and her husband and two children, were desperate to escape Nazi Germany before they too were killed. Klaus approached the family and offered to help them escape Nazi Germany by driving them to the border, in exchange for Esther's valuable family heirloom brooch. However, Klaus betrayed the family and instead drove them to Auschwitz, where they were executed and their belongings taken by Klaus.
After the Nazi regime fell at the end of World War II, Klaus evaded discovery and arrest by forging the false identity of Abraham Klein, a Jewish survivor of Auschwitz, and fled to America. There, Klaus became a clockmaker and had a Jewish son, David, and he hid and buried the truth about who he really was and what he had done for almost 60 years, until the present day.
Klaus made the fatal mistake of selling Esther Schnitzler's brooch to neo-Nazi Xander Green. When Xander recognised the brooch from Esther's journal and tried to blackmail Braun into paying him more money, Klaus murdered Xander with a bayonet handgun from his Hitler Youth days to try and keep his true identity hidden.
Eventually, the New York CSI team managed to trace the Schnitzler brooch Xander bought and then sold and the weapon Klaus used to kill Xander back to Klaus and identify Klaus' real identity. A disdainful and disgusted Mac subsequently took Klaus in and interrogated him on the truth. Klaus at first tried to deny it and insist that he was Abraham Klein, but soon the façade fell and Klaus' evil true colours were revealed; grinning evilly through the room's glass at the witnesses and saying in German, "We should have killed them all."
Klaus was subsequently arrested for Xander Green's murder and his war crimes against humanity, and his shocked and dismayed son David rejected and turned from his evil father as Klaus was taken away.