In the Timeless Universe
In the original timeline, on April 3, 1882, James, Charley, and Robert were planning a robbery, but little did James know, the brothers made a deal with Missouri Governor Thomas Theodore Crittenden to kill James and get the reward money. During the robbery planning, James boasted about his strength, and called Robert a coward. Robert shot and killed James in the back as he was dusting a painting.
In the altered timeline, Robert and Charley are shot by Garcia Flynn moments before they were going to kill James. Flynn explains to James what they planned to do. When Flynn is asked how he knew, he lies to James saying he heard them boasting about the plan at a bar.
Robert Ford (outlaw) at Wikipedia