When Young Goodman Brown returns from his experience in the forest--whether that experience was real or a dream vision--he is a changed man. He left the village the night before as a typical young married man in his village, full of life and with a pretty wife named Faith. Unfortunately, on his return his attitude toward the people of his village, as well as his wife, was completely changed--fearful, suspicious, and, more important, perceiving evil in everyone around him:
[when Goodman Brown passes his minister] He shrank from the venerable saint as if to avoid an anathema [a curse].
Goodman Brown saw, or thought he saw, his minister, Deacon Gookin, Goody Cloyse (the woman who gave him religious instruction) in the forest communing with the devil, so when he sees Goody Cloyse teaching a young girl her catechism, "Brown snatched away the child as from the grasp of the fiend. . . ." Clearly, the evil that Young Goodman Brown perceived in himself, which caused him to go into the forest at night, he sees in everyone else he once thought were good religious people.
Even Faith, whom he knows better than all the others and who tries to kiss him when she sees him in the morning after the visit to the forest, is given a cold welcome: "But Goodman Brown looked sternly and sadly into her face, and passed on without a greeting." Brown undoubtedly justifies this behavior to himself because he saw Faith with the others at the devil's meeting place.
To determine whether Young Goodman Brown's behavior is justified you have to decide if you believe one of two things: 1) Did Young Goodman Brown journey into the forest, meet the devil, and see all of the religious people from his village, including Faith, at the satanic ceremony? or 2) Did Brown, perhaps because of his own sense of guilt for having inappropriate thoughts, have a dream or vision in the forest? If you believe in (1), then you can argue that his behavior is justifiable, having been betrayed by everyone he thought he knew. On the other hand, if (2) seems more likely to reflect what actually happened to Brown, then you have to argue that Brown's sense of guilt has altered his ability to perceive reality as it is. I would vote for (2).