What literary elements in The Great Gatsby by Fitzgerald or The Kite Runner by Hosseini prove that "literature reveals what is bad in human...

This is a great quote and a great prompt to use in writing
about a lot of classic and contempory works of literature.  Most characters in
literature have a fatal flaw that causes or contributes to the conflict(s) in the story,
and there is where you can start to look for what bad elements of human nature are
revealed.  In The Great Gatsby you need to look all of the main
characters and look at their flaws -- it isn't too hard!  Tom Buchanan, Daisy Buchanan,
and even Jay Gatsby are very flawed people.  Tom is an arrogant, lying, philanderer. 
Daisy is a shallow, lying, murderer.  Gastby is a criminal who doesn't have his head
grounded in reality.  Those few adjectives are only the start of a description of these
characters!  All of the worst elements of their natures collide in the course of this
novel to create the story of Jay Gatsby and his tragic pursuit of Daisy and his
untimely death.  Tom and Daisy are careless with each other and other people's lives. 
Gastby is lost in a fantasy of Daisy that was never real and is certainly not acheivable
now.  No matter what good you can say about any of these characters, their flaws
overwhelm the goodness, thus proving the quote to be
true.

The situation is different with Amir in The
Kite Runner in that he ultimately does take actions that redeem his earlier
failures, but the novel is driven by Amir's guilt and his trying to live down his past
actions.  The novel looks squarely at Amir as a child who taunts his friend Hassan
because their social standing allows for that behavior.  The novel pivots on the key
scene of Hassan's being raped and Amir doing nothing then or later to help.  Amir's
whole life, up until the day he returns to Afganistan to get Sohrab out of the clutches
of the Taliban, is about his trying to live down that act of cowardice.  The novel
studies the aftermath of what is bad in Amir's nature, but it has a more positive end in
that Amir is able to take positive action and show an aspect of his human nature that is
caring and brave. 

It usually seems that the important
literature is tragic and reveals the bad of human nature -- probably because we learn
more from our negative experiences than we do from the good
ones.