Slim Grant was a Gunslinger in the Old West, who had an infamous rivalry with Jim "Boy" Calloway, due to Grant killing his cousin named Ged, in 1882 or thereabouts. Slim Grant says that he would have similarly shot Calloway, if he showed up.
Events of Red Dead Redemption 2
Jim "Boy" Calloway asks the player to capture Grant, and bring the man to him. The player ventures to Annesburg, where the deputy there tells him that Grant's last known location was near Beaver Hollow. Upon riding there, the player finds Grant being held captive by a handful of outlaws affiliated with the Murfree Brood gang, and following a brief gunfight, the player takes Slim Grant to Brandywine Drop, to meet with Calloway.
Almost immediately, an argument erupts between Calloway and Slim Grant about the latter killing Ged Calloway. Grant tells Calloway to move on and refuses to fight him in a duel. As Slim Grant leaves, Calloway decides to shoot him in the back, which results in Slim Grant's death.
In his book about Jim "Boy" Calloway, Theodore Levin instead writes that Slim Grant shot Calloway in the back after being mortally wounded by him in a duel.