With a reputation as a brutal sadist, he was during the Spanish Civil War responsible for the arrest, sexual abuse, torture and execution of thousands of people in the regions of Andalucia and Extremadura who opposed the Nationalist military uprising.
Wanted by the authorities, he was hidden by a female prostitute, Doña Mariquita, who later recalled the favour when she was persuaded by relatives of Diaz Criado's prisoners to help save them.
It had been overseen by the virulent extremist Mauricio Carlavilla, who had spread old conspiracy theories in the right that Jews and Freemasons were destroying Spain's past glories.
He was also believed to have been involved in assassination attempts on two members of the Congress of Deputies: the law professor Luis Jiménez de Asúa and Francisco Largo Caballero.
He took family members, including children and the elderly, as hostages and, if they remained alive, kept them in horrific conditions if the working-class leaders he was hunting could not be found, even for the duration of the war.
On 10 August, to commemorate Sanjurjada's failed coup, some murders were committed, including of Mayor José González Fernández de la Bandera and the writer and Andalucian nationalist Blas Infante.
Diaz Criado provided a daily report for Queipo de Llano; Commander José Cuesta Monereo, who was regarded as the real planner of the coup in Seville; and Colonel Francisco Bohórquez Vecina, a military judge with little interest in legislation.
Often drunk in the daytime and turning up in the evening at his office, Diaz Criado would frequent nightclubs and surround himself with admirers; female dancers; and prostitutes including Doña Mariquita, who had sheltered him in 1931.
The money and sexual favours offered by Mariquita were discussed informally between her, Diaz Criando; his friend Rebollo Montiel, who supervised most of the prisoners' torture; and whoever else was there in the small hours.
Once, some women accompanying Diaz Criado were offered a coup de grâce and shot at prisoners since they had missed seeing the firing squad in action at his random invitation.
Portugal was supporting the Nationalists and an angry Queipo de Llano, who tolerated Diaz Criado, was now a personal embarrassment and was forced to apologise.