You can still stand beside him at El Paso’s hardscrabble Concordia Cemetery, a dirt-and-gravel expanse just a stone’s throw from Interstate 10. With eighteen-wheelers rumbling by, it can be hard to concentrate long enough to read the historical marker inside the iron cage that’s been erected to protect his grave. You’d be forgiven if your mind drifts to the many stories about him instead—to the time he faced off against Wild Bill Hickok or put a bullet in the jaw of a drunken cowboy, sending his teeth clattering all over a Kansas sidewalk. He’s widely considered the greatest of the Texas gunfighters.But history has been kind, almost certainly too kind, to John Wesley Hardin, a homicidal Dixie diehard who shot his way across post–Civil War…