While we never learn exactly what compels Goodman Brown to go into the forest, we do know, at least, that his motives are not upright or pure. His new wife, Faith, all but begs him not to go that night, but he insists that he must. Then, as he walks away from his home, he calls himself a "wretch" to leave her to go on "such an errand." If he were motivated by something good, he would hardly refer to himself as miserable or to his trip in such a derogatory way. He fears, momentarily, that a dream may have "warned her what work is to be done to-night," but he assures himself that this cannot be the case. It would "kill her to think it," he believes; if she knew his purposes, then, it would kill her. Now, if he were merely planning a trip to the forest for some innocuous purpose, he would surely not have to go between sunset and sunrise, nor would it kill his wife to know what he is up to.
The most compelling evidence that Goodman Brown is motivated by sinfulness is his plan for the future. He thinks that his wife is a "blessed angel on earth; and after this one night, [he'll] cling to her skirts and follow her to Heaven." In other words, he knows very well that his resolve to be good and live righteously begins tomorrow—not now—and so it seems that he wants just one more night of sinning to sow his wild oats.