As Castro's right-hand man in the new regime, Guevara ordered the execution of hundreds of people while in charge of the notorious La Cabaña prison in Havana. He was unapologetic about the mass killings of innocent people, explaining, "To send men to the firing squad, judicial proof is unnecessary. These procedures are an archaic bourgeois detail. This is a revolution! And a revolutionary must become a cold killing machine motivated by pure hate."

"Hatred as an element of struggle; unbending hatred for the enemy, which pushes a human being beyond his natural limitations, making him into an effective, violent, selective, and cold-blooded killing machine – this is what our soldiers must become," Guevara said.

Once again, in rallying his guerrillas in Angola, he wrote: "Blind hate against the enemy creates a forceful impulse that cracks the boundaries of natural human limitations, transforming the soldier in an effective, selective and cold killing machine. A people without hate cannot triumph against the adversary."