Vendetta at Dawn(1971)

Original title: Quel maledetto giorno della resa dei conti

George Benton returns from school to his brother Jonathan's farm as a doctor and marries his fiancé Lory, but happiness doesn't last long. The Fargas brothers provide a bloodbath at the ranch and are able to divert suspicion from themselves onto a gang of passing outlaws, but one of the family has survived the massacre and knows who the actual killers are.