Bad Jim (1990)

On their way to Montana, three unsuspecting cowhands--John Coleman, July, and B.D. Sweetman--have a chance encounter and swap one of their horses with Virgilio Segura, a Mexican bandit who once rode with Billy the Kid himself. Indeed, Bad Jim, the newly-acquired mustang, was a real bargain; however, what on earth possessed the innocent companions to start killing and robbing banks? In the end, who is to blame for this instantaneous and horrible transformation?