Seetzen was born in 1906 as the only child of a deli owner in Rüstringen, in what is today part of Wilhelmshaven. While a student he joined the Jungstahlhelm. Seetzen studied jurisprudence at the University of Marburg and the University of Kiel. After his law examination he worked, helping out in various law firms. Heinz Seetzen was married to Ellen Knickrem. On 1 May 1933, he joined the NSDAP and the SA. On 1 February 1935, he became a member of the SS. After an unsuccessful bid for the post of mayor in Eutin, the unemployed Seetzen took a job as a temporary worker in the Eutin government, as an assistant to SA-Brigadeführer :de:Heinrich Böhmcker|Heinrich Böhmcker. In 1935, he joined the Gestapo. Seetzen was promoted to Chief of the SiPo and SD in Aachen, Vienna, Stettin and Hamburg. As of August 1942, he was Chief of the SiPo and SD in Kassel, and then in spring 1943 in Breslau. In 1944, he was commander of the SiPo in Prague. After the invasion of the Soviet Union in 1941, Seetzen was commander of Sonderkommando 10a, which followed Army Group South and was responsible for mass killings in the south of the Soviet Union. Austrian police officerRobert Barth, an accomplice in the mass murder, said about Seetzen: "A particularly brutal Kommandoführer . He is said to have boasted that his Kommando would shoot the most Jews. I was also told that, at his command, once the ammunition for the shootings of Jews ran out, the Jews were cast alive into a well with a depth of approximately." From 28 April to August 1944, he served as commander of Einsatzgruppe B, which perpetrated mass murder in Belarus. This unit was responsible for the deaths of more than 134,000 people in Minsk and Smolensk. After his promotion to SS-Standartenführer and police colonel, he was made Commander of the SiPo and SD in Belarus in April 1944.
Post-war
After the war, Seetzen stayed with a female acquaintance, hiding his identity by using the false name "Michael Gollwitzer". His acquaintance reported that Seetzen was remorseful and completely finished from a moral perspective. He told her "that he was heavily burdened by guilt, that he was a criminal, and that he had essentially forfeited his life." He also openly admitted that he would commit suicide by taking potassium cyanide the moment he was captured. After his arrest by the British military police in Hamburg-Blankenese on 28 September 1945, Seetzen committed suicide using a cyanide capsule. He was not identified and was buried as "Michael Gollwitzer". Due to this fact, a Denazification Court classified Seetzen as a "lesser offender" in 1949, adding the stipulation, "in the event that the person concerned is still alive".
