In the Spanish Civil war, the left was responsible

I am not partisan of Franco and his supporters, I am just point out that the biggest killers being on the left.

As for putting down an insurrection; perhaps a little history lesson is in order:-

In February 1936, the Popular Front, a coalition, won a majority of the seats in the Spanish parliament. The alliance was made up of the Socialist Party (PSOE), the Republican Left Party, the Republican Union Party and Communist Party of Spain, as well as Galician and Catalan nationalists. In total they got 34.3 percent of the popular vote, compared to 33.2 percent the opposition led by CEDA.
This was not much of a mandate.

The government the immediately released all left-wing political prisoners and started to ban opposition political parties and transferring officer not loyal to the left out of the Spanish mainland.

The true tipping point was when Calvo Sotelo, leader of the right winged monarchist party, is murdered by police officers.

When you start banning and murdering your political opposition, they tend to respond.