As you might understand (or avoid to). Franco didn't wake up one day and decided to start a civil war out of nowhere. In the period of 1933-1936, there was major violence against the legitimately elected Right-Wing government from Marxist groups that adopted a revolutionary approach to take power in Spain:
"The defeat in the elections and its consequences led to disenchantment with parliamentarism and radicalization within the Socialists. The increasing militancy within the Socialist workers was followed by Francisco Largo Caballero's adopting a revolutionary Marxist rhetoric which justified revolutionism as a way to combat rising fascism, uncharacteristic of European social democratic mainstream and the reformist traditions of the PSOE.[69] The CNT adopted a similar rhetoric in the wake of the elections, threatening with a revolution if "Fascist tendencies" would win the elections.[70] Open violence occurred in the streets of Spanish cities, and militancy continued to increase,[71] reflecting a movement towards radical upheaval, rather than peaceful means as solutions.[72] A small insurrection by anarchists occurred in December 1933 in response to CEDA's victory, in which around 100 people died.[73]
[...]
Fairly well armed revolutionaries managed to take the whole province of Asturias, murdering numerous policemen, clergymen and civilians, destroying religious buildings including churches, convents and part of the university at Oviedo.[75] Rebels in the occupied areas proclaimed revolution for the workers and abolished the existing currency.[76] The uprising was crushed in two weeks by the Spanish Navy and the Spanish Republican Army, the latter using mainly Moorish colonial troops from Spanish Morocco."
https://en.wikipedia.org/wiki/Spanish_Civil_War