A quality of someone's dark side that Brown exhibits is insecurity. First, he would rather believe others than believe himself. For example, when Brown travels with the devil, he argues that his family couldn't possibly be corrupted. The devil says otherwise, and Brown automatically believes him. Second, he doesn't admit what he thinks. He doubts the existence of heaven in his own mind, and then says "With heaven above and Faith below, I stand firm against the devil!" Third, he jumps to conclusions too easily. When the pink ribbon falls, he assume instantly that it belongs to Faith. He starts to lose his mind over a ribbon that could have belonged to anyone.	
	Goodman Brown reveals that he has too much of a dark mind. For example, he tries to resist the devil but cannot seem to get away. The devil practically vanishes right in front of him, and conveniently enough, Deacon Gookin comes down the path almost immediately, speaking of devilry. Goodman Brown does not even consider the possibility of illusion. He believes the vision because he wants to believe it. In addition to that, the demons inside of him escape. When he's trying to find Faith, he is laughing maniacally. The author emphasizes this when he says "The fiend in his own shape is less hideous than when rages in the breast of man." Also, it is not temporary. The darkness follows Brown around for the rest of his life, and he makes no attempt at ridding himself of it.
	In my opinion, the author is trying to show Brown's insanity through the devil, but I don't believe the devil exists, at least in Goodman Brown's life. It is an analogue for his own shortcomings and insecurities. Goodman Brown can't blame things on himself because of narcissism, so he creates this image of the devil to justify his life and have something to blame for whatever may go wrong. Because of his insecurity and self-loathing he put his wife on a pedestal, and is unable to accept less than perfection from her. Had the story continued, it would not surprise me to see Brown turn on her at the slightest imperfection, claiming her to be corrupted by the devil and maybe even murdering her.
	Someone like Brown believes that our deeds are controlled by outside forces. And since the devil is responsible for his woes, that means he does not have to take responsibility for his own life. He takes malevolence and puts it at arm's length because he can't live for himself. To do that would be too frightening for him, and so he tells himself there is a devil on his shoulder and that is the entity responsible for whatever happens. 
	The whole point the author is trying to make is that we are the true arbiters of our destiny, if there is such a thing, and we can't rely on these outside entities either to help or hinder us. What we do is of our own making, and this is something we have to learn through experience.
