In what ways does this parable convey the message that people posses the potential for both good and evil?

"" largely
conveys the idea that people have the potential for both good and evil through theof Mr. Hooper,
the minister, himself.

Mr. Hooper is routinely called "good Mr.
Hooper" by both the narrator and the members of his congregation.  Further, he's always
"had the reputation of a good preacher, but not an energetic one: he strove to win his
people heavenward by mild, persuasive influences, rather than to drive them thither by the
thunders of the Word."  He is no fire-and-brimstone preacher who tries to scare his
parishioners into right behavior with threats of eternal hellfire if they do not repent and
change.  Instead of threats, Mr. Hooper prefers gentle encouragement.  To him, his job is not to
scare his congregation but to urge them into right behavior with his gentleness and care.  He
builds them up rather than tearing them down.  Mr. Hooper has only ever been known as kind and
good by his flock.

However, it may be his reputation for goodness that
actually causes Mr. Hooper to feel that wearing the black veil is necessary.  In a conversation
with his fiancee, Elizabeth, Mr. Hooper reveals the veil's meaning (in a very veiled way
-intended!).  He says, 

"If I hide my face for
sorrow, there is cause enough, [...] and if I cover it for secret sin, what mortal might not do
the same?"

Elizabeth sits silent for a few minutes,
"But, in an instant [...], a new feeling took the place of sorrow: her eyes were fixed
insensibly on the black veil, when, like a sudden twilight in the air, its terrors fell around
her."  Suddenly, the veil's meaning becomes clear to her and she leaves him forever.  What
she has come to understand is that the veil is a literal, material symbol of the unseen veil
that Mr. Hooper believes each of us wears in life.  We each present to the world the aspect of
goodness -- as he has done -- and we hide our secret sins from one another, essentially lying
about our true natures, and this "veil"  of lies separates each of us from our
fellows.  We can never truly be known by another, and we can never truly know another because we
insist on hiding the very thing that actually unites us all: the fact that we are, each of us,
sinners.  

Therefore, Mr. Hooper has shown his own capacity for goodness and
for sinfulness (or evil, if you prefer) through his reputation, what people observe to be true
of him, and his decision to don the veil, an attempt to remedy the inaccurate picture painted
only by his reputation.  Knowing that what he was presenting to the world before he put on the
veil -- an image of only purity and goodness -- was a lie, he attempted to
rectify it and convey something more true than his reputation alone can ever be (precisely
because he hides his secret sins from others, as we all do): that good and evil combine in each
of us.