Don T got it right.

In the "wild west" (the movie version mostly) that's the basis/background for the song, there was no such thing as a "handsome stranger" with money in his pockets and carrying a gun unless he was part of a trail herd and/or worked for a reasonably prosperous rancher. In either case, those with whom he worked were a "brotherhood" that didn't accept any ill fortune that befell one of them if there was someone handy to blame.

Regardless of justification or legality, shooting a "person not regularly seen in town" was akin to the more modern act of hitting a member of a biker gang in the head with a shovel. Retaliation was certain, and grudges were held for a very long time.

While in the old west there was only a small bit of truth to the "lore," it is unfortunately explicitly visible among some current (street?) gangs worldwide now.

Today we'd say "He had to run 'cause the stranger wore colors," and everyone would understand.

John