How does Curley cause us to see Lennie's underlying nature?

I would say that there are two distinct moments when
Curley brings out Lennie's true nature.  It might not be deliberate, but each moment
brings out his underlying essence, something that we, as the reader, already knew, but
grasp fully in seeing Lennie in these moments.  The first would be the fight.  Curley is
pounding on Lennie and he takes it.  Lennie cowers in both pain and fear.  This brings
out his true nature because it reveals that Lennie is not maliciously aggressive.  He is
not an instigator, like Curley.  Yet, when George commands Lennie to attack him and
fight back, we also see Lennie's loyalty to George. At this instant, in following
George's words, he catches Curley's fist in his hand and it dwarfs as a result. 
Steinbeck compares Curley to a fish flopping once Lennie catches his fist.  The result
is that Lennie breaks Curley's hand, demonstrating that while Lennie has a capacity for
love and loyalty, there is also intense destruction within him.  The other instant where
Curley brings out Lennie's true essence is when he leads the hunting party to catch and
kill him.  Lennie's underlying essence as a victim, someone who will always be targeted
in this life, becomes apparent in this moment, in this instant.  Lennie's underlying
essence of being afraid and being unwanted is something that is on display, initiated by
Curley's own antagonism.  The statement and implication is that there are many more
cruel and savage people like Curley in the world than the tenderness represented by
Lennie.  It is a condition that Steinbeck displays, hoping to initiate some level of
change.