In his letter to his sister Margaret, dated August 26th, 17--, Walton tells her that he has listened to the "strangest tale that ever imagination formed." Continuing, Walton relates how absorbed he has been by this tale, and how impressed he has been by the "elevated and gentle manners" of Victor Frankenstein, who has touched him deeply.
Because Walton has heard the history of Victor Frankenstein, he amends his statement in his fourth letter that his venture is important enough that a few lives are worth the cost--"a small price to pay for the acquirement of knowledge." Instead, he now tells his sister that
it is terrible to reflect that the lives of all these men are endangered through me. If we are lost, my mad schemes are the cause.
Thus, it is because of his warm relationship with Victor Frankenstein that Walton becomes more prudent regarding his venture, listens to his crew, and turns back, heading home.