Woow~ They were all gunned down. Who...could've seen that coming?
I'm guessing this encounter was primarily designed to introduce Mr. Mercenary, which is fine. He seems to have enough of a sense of morality to not claim his bystander's right, which is a decent good start as far as interviews with the boss (Mitsuha) go.
Yeah, the 5 farmers probably would have and should have run away if the mercenaries were insta-killed. That would be the sane thing to do.
Though I'm not convinced Mitsuha wouldn't have shot their backs as they fled for some reason or another.