Beltrán Dalay Cortés Carvajal was born on November 12, 1908, in Santa Bárbara, Heredia, Costa Rica. He was the youngest of 18 siblings. His parents were Rosendo Cortés Madrigal and Amelia Carvajal. Growing up in a large family, he lived with his mother and three sisters by 1938. He went through the first levels of elementary school before becoming a farmer. Beltrán never got married and did not have any children.

As a child, Beltrán experienced a serious accident at the central market in Heredia when a door fell on him. This incident caused a head wound that left a permanent scar. He also suffered a fracture in his right arm, but it was not treated properly. Because of this, the arm never healed correctly.

When he turned 20, Beltrán had three surgeries to repair his arm. These surgeries were performed by doctors Ricardo Moreno and Carlos Echandi. The first surgery took place on June 1, 1928, when they wired his broken humerus. He had two follow-up surgeries in July and August of that year. Another procedure involved metal plate implants in 1929, but these were removed in 1932. At that time, doctors diagnosed him with an unconsolidated humerus due to syphilis.

On July 28, 1934, Beltrán Dalay Cortés Carvajal, while working as a police officer, killed his partner, Benjamín Garita Ramírez. He was convicted and sentenced to five years in prison on San Lucas Island. After some time, he had his sentence reduced by performing manual labor. Once released, he briefly worked at Chapuí Asylum but soon quit to sell merchandise on the streets.

Cortés became fixated on the surgeries he had experienced with two doctors, Ricardo Moreno and Carlos Echandi. He believed his failed surgeries were their fault. Before committing murder, he threatened both doctors. His mother expressed concern for him and sought help, leading to an admission request at Chapuí Asylum. On August 23, 1938, Cortés bought a gun and went to Dr. Moreno's house. He shot Dr. Moreno three times after knocking on the door. After that, he went to Dr. Echandi's home. There, he shot Dr. Echandi twice from outside, but only one shot hit him.

During his escape, Cortés also killed a Canadian man named Arthur Maynard and injured two others, Egérico Vargas Loría and Rodolfo Quirós Quirós. He was captured shortly afterward. Cortés was taken back to San Lucas Island, where he faced a sentence of undefined length on June 23, 1938. He was famously displayed in a small cell, attracting tourists. This continued until legal changes in Costa Rica led to his release after 32 years of imprisonment, as he had already served beyond the new maximum penalty.

Beltrán Cortés died on June 11, 1984, from prostate cancer at the age of 75 in his sister's house in Santa Bárbara.