What is the significance of the opening scene in Ender's Game? Closing scene?

The opening scene shows us a variety of things, one is
that there is a plan for this boy, another is that we get to see through Ender's eyes
the way he rationalizes hurting Stilson.  He begins to gain depth as a character as we
find him contemplating the consequences of various options and then deciding to kick him
again and again while he is on the ground.

The ending of
the opening scene is important because it shows also his loathing of Peter and his
desire to not be like him.

The ending scene is perhaps best
seen as a contrast to the opening one as it shows Ender's great capacity for empathy and
his love for the enemy he destroyed so ruthlessly.