With the federal government fully in charge of the country, the push to the West picked up a lot of steam and many uprooted people moved West with grudges against the other side. It took a while for the "microaggresion" to settle down.
Not to be glib, but in the movie "Shane", the farmer character known as "Stonewall" is gunned down by Wilson (Jack Palance) in part because Wilson doesn't like the guy's nickname.
Problem with the theory is that the James, Younger and Dalton gangs didn’t really operate in “the West.” They were pretty much entirely in what we now call the Middle West, which had been settled for most of a century by then.
***...the push to the West picked up a lot of steam and many uprooted people moved West with grudges against the other side.***
This is true. Lt Ware, in his book THE INDIAN WAR OF 1864 mentions lots of people moving west DURING the Civil War. Many of those heading west, still wore parts of Confederate Uniforms.
What about Jesse James’ involvement with the Knights of the Golden Circle?