From September 1941 until August 1942, he served as the first commandant of the Majdanek concentration camp in occupied Poland, stealing vast amounts of valuables and money from murdered Jews.
After World War I, Koch worked as a commercial manager, an authorized signatory and insurance agent and became unemployed in 1932 (he had served a prison sentence in 1930 for embezzlement and forgery).
That was due largely to an investigation based on allegations of his improper behavior at Buchenwald, which included corruption, fraud, embezzlement, drunkenness, sexual offences and a murder.
Hereditary Prince Josias investigated the case and found that Koch, as the Camp Commandant, had ordered Krämer and Karl Peix, a hospital attendant, killed as "political prisoners" because they had treated him for syphilis and he feared it might be discovered.
By that time, Koch had been transferred to the Majdanek concentration camp in Poland, but his wife, Ilse, was still living at the Commandant's house in Buchenwald.
Ilse was charged with the "habitual receiving of stolen goods, and taking for her benefit at least 25,000 RM..."[6] The trial resulted in Koch being sentenced to death for disgracing both himself and the SS.
[7] Koch was executed by firing squad on 5 April 1945[5] one week before American allied troops arrived to liberate the camp.
[8] Contrary to some claims, however, Karl's body was not burned in the camp's crematoria, as they had run short of coal and had stopped operating in mid-March 1945.
[9] Koch first married Käte Müller in 1924 and had one psychiatrically troubled son named Manfred; however, this marriage ended with divorce in 1931 due to his infidelity.