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....