He was accused and convicted of war crimes due to his involvement in the massacre of Serbian and Jewish civilians during the Axis armies' invasion of Yugoslavia.
In August 1942, he was pensioned and retired from the army due to his role in the massacres of Serbian and Jewish civilians during the 1942 raid in Vojvodina in present-day Serbia.
In supporting a Nazi-occupied Hungary, he was accused of committing war crimes against Serbian civilians in Vojvodina.
Deák was sentenced to death by hanging by the Vojvodina Supreme Court on 31 October 1946 for the mass murder of civilians in Novi Sad in January 1942, during a series of massacres in southern Bačka.
He was executed on 4 or 5 November 1946 in Vojvodina, along with Field Marshal Ferenc Szombathelyi and József Grassy.