The "dark figure" is the devil. He is trying to convince Goodman Brown that all people are secretly evil. He wants to sour Goodman Brown and turn him into a cynical and bitter man, so that he cannot see the good in others. He wishes for the young man to think that all displays of a virtue are simply false fronts.
Whether Goodman Brown is dreaming or whether he has had a real encounter with a Satanic ritual, he does come to believe that the people around him are evil. He doesn't come to believe evil is happiness, but his trust in others is destroyed. He can no longer look at them with the same eyes. He scowls at Faith, for he now believes her purity is all a sham. Likewise, his trust in his neighbors is destroyed by the conviction that they are all hypocrites.
This is not the viewpoint of the author/narrator. At the end of the story, he condemns living a life of suspicion, showing that it warps a person and destroys his or her happiness. The narrator describes Goodman Brown as having received an "evil omen" in his encounter in the woods, and says he died in "gloom," suggesting that his life was joyless from that night on. The author is saying that we need to trust the people around us and see the good in them, even if they may at times be imperfect.