Seetzen was promoted to Chief of the SiPo and SD in Aachen (1935–1938), Vienna, Stettin and Hamburg (January 1940 to July 1941, then absent until August 1942).
Austrian police officer Robert Barth, an accomplice in the mass murder, said about Seetzen: "A particularly brutal Kommandoführer [...].
After the war, Seetzen stayed with a female acquaintance, hiding his identity by using the false name "Michael Gollwitzer".
[3] After his arrest by the British military police in Hamburg-Blankenese on 28 September 1945, Seetzen committed suicide using a cyanide capsule.
Due to this fact (since his whereabouts remained uncertain), a Denazification Court classified Seetzen as a "lesser offender" (Group 3- Minderbelasteten) in 1949, adding the stipulation, "in the event that the person concerned is still alive".