What will motivate Peter and Wendy's treatment of David at the end?"The Veldt" by Ray Bradbury

Because of the technology of their house, and because of
technological advancements, Lydia and George Hadley do not function in the traditional
roles of mother and father.  Rather than engage in physical activities with their
children, the Hadley's have spoiled their children by purchasing a virtual reality
nursery for their son and daughter, Peter and Wendy.  However, the nursery has lately
begun to not only create a veldt, but to exert some kind of influence upon the children,
and perhaps even the parents as there has now been an inversion of power. When asked why
the room was in the veldt, Peter denies it, sends Wendy to check, and she returns saying
it is not so.  When George goes himself to the room, there is a lovely forest instead. 
However, when George tells the children to go to bed, they look at him with mouths
agape.

Clearly, the children have been spoiled and feel
they should be in charge.  When George tries to turn the house off, it is too late; as
the psychologist tells the Hadleys, the room has
become

a
channel toward--destructive thoughts, instead of a release away from them....You've let
this room and this house replace you and your wife in your children's affections.  This
room is their father and mother, far more important in their lives than their real
parents.

Underscoring David
McClean's words, the children are insolent and  disrespectful to George, telling him
they hate him and wish he were dead.  Unfortunately, Lydia is still indulgent, and turns
the nursery back on after George has shut it down.  Later, the Hadleys are locked in and
then learn why screams they have previously heard are
familiar. 

When David McClean arrives, it is too late.  The
children have completely embraced the veldt as their parents and home.  As McClean looks
for the Hadleys he sees the two children picnicking on the corner of the open glade of
the veldt.  In a distance, he sees the lions, who have been feeding on something.  They
move to the water hole to drink.  Just then, a shadow falls upon McClean's face and
vultures start dropping from the blazing sky.

Having rid
themselves of the nuisance of the parents, Peter and Wendy face Dr. McClean now.  It is
he who suggested that George shut down the veldt. So, Wendy invites him to have some
tea, hoping to delay him in order to dispose of another conflicting force against hers
and Peter's desires.  The lions will feed again.