Mr. Padilla wanted to join the army. He liked the uniform and military life and had no interest in politics. He is friendly and chatty, but his eyes well up with tears when recalling events.

Days after the coup, he recounted, a lieutenant in his regiment, Aníbal Barrera, picked a group of conscripts to be part of a firing squad. “We didn’t want to go, but he shouted and insulted us and threatened that if we didn’t comply, we would also be killed,” Mr. Padilla said.

A prisoner was thrown face-down on a truck, and the officer and soldiers took him to La Ballena Hill in Puente Alto, a few miles away. The prisoner was not blindfolded, but was placed with his back to the firing squad. Then the soldiers were ordered to shoot.

Mr. Padilla’s account corresponds with records of the execution of José Rodríguez Hernández, who was arrested by the police on the street carrying Marxist books, handed over to the regiment and killed at La Ballena on Sept. 14, 1973. Decades later, Lieutenant Barrera and the commander of the regiment, Colonel Mateo Durruty, admitted to the killing. In 2011, Mr. Durruty was sentenced to four years on conditional freedom. Mr. Barrera was not charged.

Mr. Padilla was not identified as a member of the firing squad, and has never been summoned to court to testify about this or any other crime. He has spent decades trying to convince himself that he is not a murderer.

“I’ve fired at people, but I can’t say I have killed because I don’t know if my shots were the ones that killed,” he said. “Or I just don’t want to believe it.”

He added, “It’s been eating away at me all these years.”

When he was chosen for the mission in the south, which he described as a “death caravan,” Mr. Padilla and other soldiers were given special training in the Buin Regiment in Santiago. One day, as the conscripts were watching a movie, he recalled, officers led a soldier to the patio and shot him. The conscripts were told the soldier had been passing classified information to the communists. “They killed him there so we would all see what could happen to us,” he said.