I'm citing evidence from The Pearl about this theme: People are never really free--their lives are determined by outside forces.
Steinbeck illustrates this philosophy, naturalism, allegorically early in Chapter 1, describing an ant trying to escape a trap while a detached God watches.
Among the forces that control the lives of Kino's people are imperialism, illustrated by the doctor in Chapter 1 who considers them animals, and ignorance, which puts the people at the mercy of those like the doctor - "...he could not take the chance of pitting his certain ignorance against possible knowledge...he was trapped" (Chapter 3). Religion also is a controller, with clergy who treat the people like children, refusing the marriage sacrament if a couple can't pay, and teaching of "gods who do not love men's plans...do not love success unless it comes by accident" (Chapter 3). If a man's plans go well, it is because "power was given to him", and if they go bad, it is God's punishment because man "rebelled against the way things are" (Chapter 3).
Finally, man is controlled by human nature. Riches would "graft unto (man) the evil limbs of greed and hatred and coldness" (Chapter 5). Because of all these forces, Kino becomes "an animal" (Chapter 5) - a "terrible machine" (Chapter 6) who kills his own son with the rifle he coveted.