Another element of the theme of good and evil to consider is found toward the end of the story, at the end of the witch meeting in the woods. The Devil is speaking to the congregation gathered there and is speaking of a variety of evil acts such as wives murdering their husbands, sons killing their fathers, and young women killing unborn children. His list of "secret crimes" is shocking, and it is this kind of knowledge, to "see the inmost secrets of man" that he promises to those who follow him. By this point in the story, Goodman Brown has had his faith in the goodness of others pretty well destroyed, and while he is tempted to join, he rejects the offer.
While this act seems as if it would save Brown, it is revealed in the end that Brown never looked at his neighbors and townspeople the same way after that night. Even though he rejected the gift of "sight" he still ends up being able to see what he now preceives as everyone's inherently evil nature. He can see no goodness anymore. Brown sees himself as superior to everyone else, but his subsequent behavior towards everyone alienates him and destroys his goodness. He is an embittered man at his death and the people have no kind words for him -- not because they are evil, but because he could not see their goodness or their humanity. I don't think Hawthorne is suggesting that man is evil, but that we have a balance of good and sinfulness. Hawthorne is critizing the hypocrite in Brown, not the people of the village.
Clearly, the theme of good and evil and the rather porous boarder that lies between these two states is a key theme of this tale. One way of tackling this theme might be to look at how the Devil interprets the past actions of Goodman Brown's family.
Like so many of Hawthorne's short tales, this story is rich in allegorical overtones, in that it is clear that the characters and actions stand for abstract qualities. As Goodman Brown sets off on his journey into the woods, a "fellow-traveller" journeys with him, who it is clear is the Devil. The journey into the forest itself has allegorical significance, as is made clear when Goodman Brown responds to the invitation of the Devil to go deeper into the woods:
"Too far, too far!" exclaimed the goodman, unconsciously resuming his walk. "My father never went into the woods on such an errand, nor his father before him. We have been a race of honest men and good Christians since the days of the martyrs; and shall I be the first of the name of Brown that ever took this path and kept-"
It is clear then that Goodman Brown is proud and convinced of his own "goodness", as represented by his name, for he, in his own imaginings, at least, is a "good man." The journey into the woods, therefore, is representative of engaging with evil. However, note how the Devil responds to this protestation of goodness:
"Such company, thou wouldst say," observed the elder person, interpreting his pause. "Well said, Goodman Brown! I have been as well acquainted with your family as with ever a one among the Puritans; and that's no trifle to say. I helped you grandfather, the constable, when he lashed the Quaker woman so smartly through the streets of Salem; and it was I that brought your father a pitch-pine knot, kindled at my own hearth, to set fire to an Indian village, in King Philip's war. They were my good friends, both; and many a pleasant walk have we had along this path, and returned merrily after midnight. I would fain be friends with you for their sake."
Key to this story, and so many of Hawthorne's tales, is the idea of the darkness of humanity, however sinless it pretends to be. Hawthorne makes it clear that sin touches everyone, including the supposedly "Goodman" Brown. You might find it interesting to compare this short tale to another one of Hawthorne's gems: "The Minister's Black Veil", which likewise touches on this theme of the universal sinfulness of humanity. Hope this helps you with your talk. Good luck!