. Ultimately, why does Ethan decide that it is impossible for Mattie and him to escape Starkfield and go west?

Etthan is, whether he likes it or not, an honorable man. He does not have it in him to abandon Zeena, or deceive the Hales. His choice to live honestly rather than pursue his passion for Mattie is admirable, but it should not be taken as evidence of human agency. Ethan simply does not have it in him to take money hrom the Hales and flee; this is not part of his psyche. Ethan's decency is part of what makes him so sympathetic, but in Wharton's universe goodness and morals limit options rather than multiply them. His decision to remain honest is one of the few choices he is allowed in the novel, and it lends his character tragic dignity. It is not a sign of man's power over his own life; rather, it is an example of a man behaving honorably within the constraints that life has given him.