It’s been more than 80 years since Clyde Barrow and Bonnie Parker charged through the back roads of Texas, Louisiana and Oklahoma, leading a trail of stolen cars, robbed banks and dead bodies in their wake. But time has only served to make the brash deeds of Bonnie and Clyde more notorious—and a letter from Barrow being auctioned next month paints a chilling picture of the duo’s criminal malice.

In an auction at RR Auction set to take place September 15 through 25, bidders can vie for an April 1934 letter from Barrow to an imprisoned ex-member of his gang. Written in the handwriting of Parker, the letter was sent to Raymond Hamilton, who was serving time in the Dallas County Jail. It is expected to go for over $40,000.

As the letter demonstrates, Parker and Barrow grew to hate Hamilton after he betrayed them in early 1934. Once a member of the infamous Barrow gang, Hamilton eventually earned the contempt of his partners in crime. In early 1934, Barrow and Parker orchestrated a prison raid and freed Hamilton and four other convicts from the Eastham Prison Farm near Houston, Texas. Hamilton had been imprisoned at the farm, which was also known as “Bloody Eastham” for its brutal treatment of convicts, after killing a sheriff with the Barrow gang.

The daring raid freed Hamilton, but he fell out of favor with Bonnie and Clyde after displaying what they thought of as cowardice during a police encounter. Hamilton eventually left the gang after clashing with its leaders about money and his girlfriend and was promptly recaptured by Texas police. This time, he was sentenced to death.

Unable to keep their hatred of Hamilton to themselves, the gangsters wrote to him in prison. “I should have killed you,” wrote Barrow. “[T]hen I would have saved myself much bother and money looking for you.” The letter enumerates the pair’s many complaints about Hamilton, whom they called cowardly, dirty and boastful.

Bonnie and Clyde did not live long after Barrow wrote this letter: Less than a month later, they were killed in a hail of bullets when a posse of police officers encountered them on a Louisiana road. In the letter, Barrow correctly predicted that he would one day be killed, but he was wrong about one thing when he wrote that “it won’t be without resistance.” In reality, Bonnie and Clyde had no chance to resist: When ambushed, they were shot over 150 times. Both died before they could pull out their own guns.

“The law had settled the score with Barrow and his quick-shooting woman accomplice,” reported The New York Times—but not before Barrow let his onetime friend know how he felt.