Goodman Brown’s feeling of “brotherhood” with evil, the “sympathy of all that was wicked in his heart,” seems like a significant part of...

Ultimately, I believe young Goodman Brown's willingness to step forward in this passage is internally driven.  It is true that he is struck by the fact that people he'd believed all his life to be pious and righteous individuals sit together, side by side, with known criminals and sinners.  However, "he was himself the chief horror of the scene" because "The fiend in his own shape is less hideous, than when he rages in the breast of man."  In other words, the devil himself is far less frightening and awful than when he prompts men who might have chosen to be good to do evil.  This propensity toward sinfulness already existed in Brown's nature.  Remember, he chooses to enter the forest and meet with the Devil, thinking he will do so on just this one more night and then he'll be good and "'cling to [his wife, Faith's] skirts and follow her to heaven.'"  He is only too ready to lay down his faith, believing that it will be there for him later.  He puts it down, long before any external motivators present themselves.  Brown chooses to embrace evil, even if for a short time, and so his internal motivation is enough to compel him to step forward later.