In "Young Goodman Brown," Satan says “Evil is the nature of mankind.” How is this true?
Brown's walk through the woods brings him in contact with many people he knew from the town, all seemingly heading for the "black mass" that comes later. Hawthorne describes the first person other than his guide that he meets: "As nearly as could be discerned, the second traveller was about fifty years old, apparently in the same rank of life as Goodman Brown, and bearing a considerable resemblance to him, though perhaps more in expression than features. Still they might have been taken for father and son." This and other references include Brown's family in the woods. Of course, Hawthorne never really tells us ... he "might" ... There are many others he meets: Good Cloyse, Deacon Gookin and even his Faith. Of course we never know if these people are there, because we never know if the journey actually happened. The Devil would like Brown to believe that, since they are all there, Evil must be the nature of mankind. What we know from their behavior in the town is that they are not evil ... simply human, less than the perfect that Brown demands of them.
If evil means imperfection, then evil is truly the nature of mankind. I suspect you'd have to clarify what is meant by evil.
"The enemy of the good is the perfect."
Well, the idea of mankind being basically evil is based out of communities who are led by a spiritual leader, such as it happens in Young Goodman Brown, and their need to consistently remind themselves to follow the righteous path. This is so that their souls, which are stained by the original sin of Adam and Eve, do not follow a tendency to fall into temptation. In the story this is evident in that Goodman Brown falls very quickly under the spells of the forest. It took almost no time from the time Brown leaves his wife, aptly named "Faith", and the end of his path, where he sees the hideous visions. So, basically, Young Goodman Brown is demonstrating the susceptibility to weakness that we all possess.
Arguably, all of Hawthorne's fiction tries to establish the way in which all of mankind is tainted with evil that lies under a thin veneer of hypocrisy and social respectability. This is the true message of this excellent story, as Goodman Brown has his assumptions about people he has known all of his life deliberately confused and played with. Thus it is that Hawthorne points towards the way in which none of us are perfect, as #2 indicates, and that we are all capable of evil. To pretend otherwise is hypocrisy. This is the truth that dooms Goodman Brown to a joyless existence after his night in the woods.
This statement from "Young Goodman Brown" is Satan's voicing of the Calvinistic tenet of the depravity of man. Puritans believed, as did the Calvinists, in the Divine predestination for the elect; all others were evil and damned. Since Goodman Brown counts himself among this elect, he rejects any other idea and concludes that this evil/depravity is in the others, rather than in himself.