An important figure and board member of the Jewish group in Vienna during the early stages of the Second World War, he was also an "Ältester" (council elder) of the Judenrat in the Theresienstadt concentration camp after 1943.

[1][4][5] As a board member of the IKG,[3] he began after the annexing of Austria into Nazi Germany producing reports which were presented to Adolf Eichmann as part of that man's work emigrating and seizing the property of the Jews.

[1][3] In his interviews with Lanzmann decades later, he revealed that he had observed Eichmann, armed with a crowbar, organizing the destruction of Vienna's Seitenstettengasse synagogue during the Kristallnacht pogrom.

He worked with the IKG to help over 125,000 Jews leave the country by 1941, but in that year the Germans began closing the borders as their emigration policies were evolving towards those of internment and extermination.

Nor was that the only behavior on Murmelstein's part that caused fellow prisoners to fear and revile him,[1] leading him to be nicknamed "Murmelschwein" (conflating his name with the word for "pig").

[10] He also purportedly refused to grant exemptions for deportations to Auschwitz unless a substitution was offered, and he was alleged to accept bribes to keep people from being added to those doomed to that fate.

[1][9] Whatever his actions and his motivations, for Murmelstein the liberation of Theresienstadt did not immediately lead to lasting freedom, as he was quickly detained by the Czechoslovak government on suspicion of collaboration.

After Murmelstein's death, these interviews would become the basis of a 2013 documentary, The Last of the Unjust, which raised considerable attention to him and invited extensive evaluation of his role.

[1][4] In his anthology of the classic writer, Murmelstein wrote that the "divided and ambiguous nature [of Flavius] turned him into a symbol of the Jewish tragedy.