Julio Santana dropped to his left knee and propped his right elbow on his hip, holding firm his hunting rifle until he had the man known as Yellow in his sights.

It was Aug. 6, 1971, and Santana was 17 years old.

In his village, deep in the Amazon rainforest where he lived in a hut with his parents and two brothers, he was known as a good shot. But he had only ever hunted forest rodents and monkeys for food. The man he was about to kill, Antonio Martins, was a 38-year-old fisherman with blonde hair and fair skin. Julio had been watching Yellow under a stiflingly hot forest canopy for three hours, and now wasn’t sure he could actually bring himself to pull the trigger.

Yellow had raped a 13-year-old girl in a nearby village, and her father had hired Santana’s uncle, a professional hitman, to kill him. Julio knew that in the sprawling and lawless Amazon, locals had taken the law into their own hands for hundreds of years. Still, he was shocked to find out that his favorite uncle — a military policeman — was also an assassin-for-hire. And now he was passing on his latest assignment to his nephew, hoping to recruit him as a contract killer.

Santana was reluctant, fearing that he would go to hell for killing another human being, but when his uncle, Cicero, explained how Yellow had tricked the girl, promising to take her to see the pink dolphins on the Tocantins River before raping her in his canoe, Julio began to change his mind.

To seal the deal, Cicero, too sick with malaria to do the hit on his own, told his nephew God would look the other way. All it took was 10 Hail Marys and 20 Our Fathers after the murder, he said.

“That way I guarantee you will be forgiven,” said Cicero.

Gripping his rifle, Santana stared straight at Yellow’s chest as he stood in his wooden fishing boat in a clearing near the river. He knew that at just 40 yards, he couldn’t possibly miss his target. When the shot rang out in the stillness of the forest, Santana saw a fleeting look of terror cross his victim’s face before he fell dead into the bottom of his boat. Later he would get rid of the body, gutting his victim and throwing him into the river where schools of piranhas would devour the remains.

“Never in my life will I kill anyone, Lord,” he said. “Never again.”

Santana would remember that first kill for the rest of his blood-drenched career.

Even after he had taken nearly 500 lives to become the world’s most prolific hitman, the look on Yellow’s face in the moment before he died would haunt his dreams for decades.

Santana had few aspirations in life. Like most young men in the Brazilian hinterland, he seemed “destined to become a peaceable fisherman lost in the depths of the rainforest,” writes award-winning Brazilian reporter Klester Cavalcanti in his new book “The Name of Death,” which chronicles Santana’s career. In Brazil, the book has also been adapted as a feature film.

Cavalcanti said he came across Julio on a reporting trip to the Amazon 10 years ago to investigate modern-day slave labor.

“A federal police officer told me that it was very common in that region for ranchers to contract hired hit men to kill fugitive slaves,” Cavalcanti, 49, told The Post. “I told the officer that I would really like to interview a hitman and he gave me a number for a pay phone and told me to call it at a certain date and time.”

When Santana answered the pay phone in Porto Franco, the small town in the outback Brazilian state of Maranhao where he was living at the time, he was reluctant to speak to the reporter.

“I spent seven years convincing him to talk to me about his life,” Cavalcanti said. “We spoke about everything and not just about his job. He spoke about his childhood, his relationship with his parents and his brothers and the quiet life he lived in the forest as well as the internal drama that he faced when he started to work as a hired killer.”

For his part, Santana, now 64, told The Post in an interview over e-mail last week that while he was pleased with the “honest” way in which Cavalcanti told his story, he was less pleased with the film that seemed to glamorize his profession.

“The true story of my life is much sadder than anything you can imagine,” he said.

After the first kill, Santana’s uncle offered him up as an assassin for the Brazilian government in its battle against communist insurgents in the Araguaia River basin in the Amazon. From 1967 to 1974, the so-called Araguaia Guerrillas tried to establish a rural stronghold in order to topple Brazil’s military dictatorship, recruiting farmers and fishermen to their cause.

In the early 1970s, Santana was contracted first as a guide to track down guerrilla encampments. In one case, he helped capture leftist militant Jose Genoino, a law student and one of the guerrilla leaders. Santana watched in horror as soldiers spent days waterboarding him at a secret location in the rainforest. Years later, Genoino became a congressman and president of the left-wing Worker’s Party. In an interview with Cavalcanti he remembered the “boy” in the group who had captured him in the Amazon. Julio was barely 18 at the time, and was partly rewarded for his work with a bottle of Coca Cola — his favorite drink and a luxury that his impoverished family could never afford.

Shortly after the Genoino capture, Santana shot and killed another communist militant, a 22-year-old school teacher named Maria Lucia Petit. For almost two decades Petit was simply listed as “disappeared.” The full story of how she ended up in a mass grave in a dusty cemetery, her body wrapped in an old parachute, only recently came to light after her family pressed a Brazilian truth commission to exhume bodies.

After civilian rule was restored to Brazil in 1985, Santana’s victims turned from political targets to larcenous wildcat gold miners and cheating spouses. In 1987, after he killed a married woman suspected of having an affair, Santana was caught by local police and spent a night in jail. He was released after giving up his new motorbike as a bribe.

It was around this time that Santana says he discovered his uncle was cheating him by arranging the hits but only giving Santana a tiny fraction of the amount he was being paid in advance. On average, Santana says he earned between $60 and $80 per hit, which in the years he was active would have been equivalent to a monthly minimum salary in Brazil. After he confronted his uncle about exploiting him for more than 20 years, he never spoke to him again, he said.

Santana stopped dealing in death in 2006 when he turned 52 and after his wife gave him an ultimatum.

“Either he gave up that life or he could forget her and their children,” writes Cavalcanti. “His wife repeatedly told him that his ruse of saying 10 Hail Marys and 20 Our Fathers, which Julio continued to do after every murder, was not proper repentance.”

Santana, who had been raised a Catholic, turned to an Evangelical cult to help him reform his ways.

“I have always believed in God,” he told The Post. “I believe that God gave me the strength to endure everything I suffered in my life because of that evil job. I know what I did was wrong.”

He said he has never told his two adult children or his own parents, who have long since passed away, about his career. He credits his wife, whom he met while she was working as a waitress at a bar in the Amazon, with encouraging him to leave his line of work and embrace their faith.

“She is the love of my life, the person who has given me strength to overcome everything I have been through,” he said. “Without her, I would be nothing.”

Today, he lives quietly in a town he won’t name in the Brazilian interior. He refuses to have his full photo taken because he says none of his neighbors know about his past. He and his wife now own a small farm where he grows vegetables, he said.

At one point in his life, he took meticulous notes of each kill in a school notebook, writing down who had hired him, where the hit took place and how much he was paid.

After he got to the number 492, he stopped logging the deaths.

“I don’t like to think about it anymore,” he said. “That part of my life is over.”